http://www.husng.com/content/husng-variance-calculator

It is really up2 you in most part.
I honestly don't believe in something like avoiding risk of ruin:) If you will set up for yourself minimum of 20 BI for level and optimum for lets be nity - 50BI. You will be in a spot when at the time reaching -30BI which is FAAAR more than normal variance in regular speed:) you will just step back into 3$ BI still having 50BI for your optimum ;) - this is avoiding risk of ruin:)
Maybe someone here will back you up with mathematical theory here, like Ry did with hypers calc, but I believe that this is a simple BR managment thing - not a variance (that can only explain you why did you lost 10BI in a row- but there could be more reasons than variance;)), but if you will have ROI = 8% I honestly think that you don't need to worry too much about ruin if this is at large sample! (like 750-1K tournaments).
If you like to read boring things about variance you can jump at the 2+2 thread: http://forumserver.twoplustwo.com/58/heads-up-nl/theory-amazing-bankroll... - its about cash game but still thinking about variance its up 2 date I suppose.
Myself honestly I have sometimes as much as 8-9 BI's downswing so If you really like to be consistent and safe try to set up your steps in BI's.
Ex: starting at stake with 30 BI's (or higher if you feel you need to) - Go up with 30BI's of your new stake (ex. 30BI of $5 = $150; 30BI of $10 = $300 so you go up with $300 BR) and step down with 30BI of lower level (in ex: 30BI of $3 = $90).
So in short EXAMPLE only:
3$ - $90 - $150 - step down at $30 (but still playing when BR = $31-89)
5$ - $150 - $300 - step down at $90 (but still playing when BR = $91-149)
10$ - $300 - $600 step down at $150 (but still playing when BR = $151-299)
20$...
 
Some of players like to stick with 50BI of old level and play only at higher level with everything above:) - but If you like to be consistent you should definitly make yourself something like above but in your own comfort zone:)
Hope it helps.:)