So I recently designed a spreadsheet that keeps track of my cash game results. I have two questions for you:

1) how many hands is a good indicator of my game? Currently around 2k hands, I have been consistently playing winning sessions but I also know that part of it is due to the fact that I've been running good, not too many coolers, various bad beats but I know poker can be more cruel than this.

2) in the spreadsheet, I calculate the winrate for my session, and in another box I have the average of these winrates. However if I, for example, win 100$ in 200 hands in a session, then lose 99$ in one hand in another session, my average winrate becomes negative despite my overall results being positive. Any idea on what I'm doing wrong, formula-wise?