sng bankroll

Okay, i've been playing sng's now for like 3 years. I started on these forums about 2 weeks ago, and let's just say i'm realizing how much of a rookie i actually am. Needless to say, i know basically nothing about percentages as far as return on investments and so forth. All i knew was i was profitting. I'm starting to learn now. i was a regular 215 sng player, and while this wasn't my full time job i took most of my money out and spent it and had a good time. I find now that as i start all over again, at these lower limits, they're harder to beat. Naturally i don't want to step up in money til have built my bankroll up. But i do have a few questions to those out there. First, where can i learn about tracking my return on investments and so forth? Second, does anyone else find the lower limits more difficult to beat than the higher limits. As i'm reading these forums and learning more, good possibility i was very lucky for a long time, where as i thought i was a good player, seems unlikely now. There's a lot to this game that i didn't know about. So now i turn to u guys for advice and direction. Where do i learn more about these things. And what kind of bankroll do i need to start stepping up in limits?