I use an IPOD touch for my ball. calculator. I figure, it's something i'm always going to have with me while traveling anyway. I use Ballistic FTE for software, the HUD (heads up display) is really easy to use. There are other types out there that are just as good or better i'm sure.
I would suggest you look at Shooter and Ballistic FTE. The are the 2 best IMO. I chose shooter and am really happy with it. Easy, accurate, I like how it stores rifle and bullet data, loads of features but many can be turned off if you want a simple output.