I may be a little nitty, but I like a 50 BI rule for cash. I found one thing that really affected me early on when building a bankroll was playing with "scared money." For me, scared money was when I did not have a big enough cushion to handle the normal variance of the game. Now for some, a big enough cushion is 20 BI (I think 20 BI is a common rule of thumb that most will agree upon). For me, I needed 50 BI to confidently play my 8-10 table sessions.
So you need to find that buy in comfort zone, preferably 20 BI or above, and go from there. Then, in regards to what I did, as I got above that 50 BI mark and closer to the next stake level, I started to mix in a few of the games at the next level and gradually transition to 100% of the higher level when I reached 50 BIs for that particular game. You just need to make sure that you are able to move down as easy as you are able to move up in stakes if you hit a downswing.