Variance depends on field size, so you cant be as aggressive with your bankroll in MTTs as single table SnGs. It also depends on winrate and how large swings, you are willing to tolerate? Are you willing to move down or pop more money into the account after a downswing? In that case you can be more aggressive.
A simple system, which has worked well for me, is to generally not play an MTT, which cost more than 1% of my bankroll. So if for instance my bankroll is 1.150$, I will play a mixture of 3,3-11$ MTTs and maybe occationally chill out with a 2,2$ MTT or take a "shot" at a 16,5$ MTT. In this way I keep my average buyin around 0,5% of my bankroll, which make huge swings to my roll somewhat unlikely. Going broke is not possible with this system, if I am a winning player, since I will automatically move down, long before that happens