It depends on if you can replenish your bankroll through another source of income and that $500 initial deposit is not your only deposit. But I think a 75-100 buyin rule is sufficient since you're a decent player and  should occasionally takes shots. Good luck.

gor for 100...u ll get less affected ...waht do u think is the best?  4 hours session at 30s...or 8 hours at 20s... i m sure u have plenty of time ...now take the right decision and be a brave

Estimate your ROI at each level (be realistic! not too high!) and plug it into a variance calculator.You can see, for example, given a certain ROI what is the minimum number of buyins you need to have a 99% chance of not going bust.

It sounds like you move up and down a lot.20 buyins seems a bit thin for you, but if you can move down fast when going below 18 buyins or so at your curent level (which would give you 36 at the level below, maybe move down at 27 buyins at that level if you keep losing) it can be a good bankroll strategy.Just make sure you have enough buyins to be plenty rolled for the $5s if you downswing hard, thereby not risking going broke (if you can redeposit there may be more optimal br strategies for you that are more aggressive).If the thin bankroll is impacting you mentally it is likely impacting your game and in turn results, and in that event I would suggest playing with more buyins in your roll.  100 seems a bit high unless you're really uncomfortable, closer to 50 would seem more reasonable.

Hey Roamus,You could try progressive bankroll requirements, like: Move up to the 33s when you have 30 BIs for it (1000$), then to the 55s when you have 40BIs (2200)... that way you can try to move quickly through the lower stakes where you should have a huge edge, then be more conservative when you're playing higher and it matters more to you.

Hey Roamus,
I believe I started with $150 at the $5s, got below $100 before I started winning.  I got to the $10s then switched to turbo $11s.  I recall using around 30-35 buyins here and losing a few after rake over 100 games.  It was depressing at the time, looking back.  Then I kept moving back from $10 to $20, $20 seemed so hard to me, I didn't listen to the few people that were saying they were the same levels.  Think about that last point before you read on, especially for those of you at the $5-20 levels.
Then an old buddy (dboy23/lolbillfrist) convinced me to try $11 turbos again.  Heater/upswing/learning to properly value bet led me to cruise through there.  I moved to the $22s and same thing, just cruised.  $33s?  It was the easiest level yet, something like a 25% ROI over 200-250 games and I was up to the $55s.  I don't think I spent more than 200 games there at the time either before moving up to the $110s.  I mostly stayed at that point, winning, but nowhere near what I expected (in hindsight I wasn't even doing badly, a few % ROI in the $110s 6 months after taking up the game is nothing to be upset about, but since I had experienced such quick success from $11-55 I really thought I could achieve 10-15% ROI in the $110s... lol).
So that's how I came up, 30+ buyins mostly after I got out of the $5s.  Like I said, I experienced some frustration at times, I really didn't want to move up initially just because I wasn't having success at the tables.
All that said, I had a full time job as a business owner for most of this time period (aside from winter months) and I didn't cash out a single time for my first year or longer of playing.  It was basically like a long term investment, so to speak for me.
But back to that point I highlighted earlier.  For you lower stakes guys, there is almost no difference from buyin to buyin.  Yes, $5 to 50 is a difference, but there are 3 levels between that have very slight increases in skill.  If you can beat something for 10% ROI, there's no reason not to think you can beat the next level for nearly the same (especially factoring in improvement).  But you see it all the time and I experienced both ends of it.  When you're losing you get way too intimidated about moving up and your own skill level.  Nobody is really as bad as they think when they feel this way.  And on the flip side, just because you had a 10% ROI for 500 games doesn't mean that's your true winrate.  When people experienced positive luck they tend to think they are finally receiving their fair share, rather than being honest and realizing how well you're running (playing well is a part of it, but it's easy to make decisions when everything holds or you keep getting great setups... try playing 2x the amount of weak pairs vs lags and receiving half the amount of strong hands you normally do, then lets talk about how good you are :), the point is you're never as good as you think when you're doing well and you're never as bad as you think when struggling).
The entire lesson points to focusing more on your decision making and improvement.  You'll likely hit some bumps along the way.  You see the guys with straight up graphs, but honestly most of the time they are burning money by not challenging themselves and moving up (a guy with a straight up graph at a double digit buyin for 1k+ games, outside of very consverative management due to things like kids to feed or massive borrowed sums to pay back, is the type I'm talking about) and soon enough they start to breakeven, can't handle it, lose confidence and their lack of improving and challenging themselves over the last year or two results in long term struggling (in exchange for short term profit).
Don't strive to have graphs like other people or results like other people, just improve your game.  Don't look at your results daily, even good players that win every month will have a losing day over a third of the time and that's not healthy to look at mentally.  Good players have winning months over 75% of the time, a much healthier ratio to look at (and one more indicative of true skill).
Getting caught up in the luck and variance of the game will only hurt you.  If you need to know about unknown concepts to feel comfortable, there's plenty of stuff out there about variance (the variance calculator can give you a nice idea), but spend a day learning about it rather than posting or talking and obsessing about variance every day, trust me it's worth it to focus on your play instead of results.

Roamus,  I try to talk on here so that everybody can receive the benefit of advice.