For mild loads you can't beat Trail Boss, and they're formula for figuring charges is simple.
Lay the case on it's side with a bullet directly underneath it. Make sure the crimp groove is even with the case mouth.
Mark on the case where the base of the bullet reaches, a Sharpie works well. Fill the case with TB until it reaches this mark and weigh it, this is your max load. For your starting load reduce your max by 30%.
Trail Boss was designed for exactly the type of thing you're trying to do. It's a very bulky powder that does a great job of filling the case to eliminate position sensativity problems and provide consistant ignition. I've used it in .3 sp, .357 mag, and .45-70 with great results.
GH1