you can use the bank roll rule of 10. Usually your roll needs to consist of 10 big blinds. If you currently play .05/.1 you will need 10x the max buy in (which is 10 bucks). So you should have started with $100 bucks. If you wanted to move up to .1/.25 you'll need 10x the max buy in of 25 bucks ($250). Hopefully this equation will help you out when you want to step up limits. If you are conservative with your money, you want do the rule of 25. I recommend the rule of 10 at low limits.