[rikz]

Buy-in for the full amount at a limit where you are comfortable losing 10-20 max buy-ins in a week without worrying about anything other than "What are the leaks in my game that are causing me to lose so much?" Odds are that you probably will do fine and not actually lose so much. It's more likely you'll play 20K hands and ask, "Why am I only winning X BB/100 hands instead of doing better...?"

I wouldn't buy-in for less than the full amount because your game plan changes quite a bit if you're playing short-stacked. Since you're trying to learn how to play NL, get in and play on all streets - not just preflop and flop (which is pretty much it for a short stack).