Just throwing out there that there is no substitute for sending rounds downrange, and recording the results. I've found that's the only ballistic calculator I have real confidence in.
Pay for the Strelok. It is exceptional.
I emailed Horus the other day to see when they would move Atrag over to Iphone / Android and they indicated that they have no plans to do so. Running on Windows mobile PC which is nearly 15 year old technology is amazing. Get with the times or go the way of Blackberry.