I would suggest to start of at the conservative end i.e. $25NL for a BR of less than $2k but $50NL for above that. Reason being is that you are bound to get de-stacked even when entering the pot as favourite. I cannot count how often my AA got beaten by lower cards when AI pre-flop or straights on the turn get beaten by boats on the river etc.

I jumped up to the $50NL level with only a BR of $500 but I lost 5 buy-ins within a week when I put my money in the put while ahead!!! Just playing solidly alone doesn't cut it a lot of the time. Now back at $20NL I was able to make most of it back because at this level I can play aggressively and with confidence.