Yep 50ppm is a great target to hit. Take into account how long you wish to age the wine, and add more accordingly. I think a corked bottle of wine will lose about 20-30ppm per year. But don't quote me on that as I'm going from memory on that.
So, if you are going to age over 1 year, about 75ppm is a good target, and if you will consume early, 50 is great!
EDIT: Oh yeah, 50 is the magic number for prevention of bacterial infection.Edited by: Dean