I've been helping a buddy tune his setup. He's new to traditional archery and didn't have a clue what arrows to get. He went to a well known archery shop in New Mexico, where he works occasionally. He's shooting a 55@28 Viper longbow, but only draws it 26". They put him in some GT 3555 with 100 grain heads and the arrows cut to 27.5". He was thinking that he was just a terrible shot and that his bow was supposed to be loud. I gave him some 250 grain field tips to try and his groups instantly shrunk to about a 1/4 of what they were. His bow really quietened down, too. He's now really excited about shooting his bow and is really improving. Before, the setup was so unpleasant to shoot, he never did.
So, don't trust Pro shops that don't specialize in trad. Stu's calculator can get you in the ballpark, but perhaps the best way is to call one of the sponsor's here who deal with these decisions all of the time.