What kind of Sit and Go did you played? Single table? 45 man? 180?
If you want to play single you will face less variance and you can have a smaller banroll to play $3.50 and $5, I dont like the way that you went from 0.50 straight to 5 though, take small steps, go up to 1$, $2. For single table sit and go's I would say 35 buy ins is the minimal and you can go up to 100 to be safier (it also depends how many tables you will play per day, if you are going to play 10 sngs a day for example you will need a much bigger bankroll)
For multi tables sng such as 45/90/180 I like to have at least 70 buy ins on regular speed, turbo and hyper-turbo you need to have more than that as well.
Whenever you want to cauculate how much bankroll you need just to the math, how many do you want to play per day? Define your daily poker cost, than calculate how many days of the month you will play, and there you have it, this is what your bankroll should be.