After several tries at the perfect shirt, I ended up somewhere between the "form" and "medium" fit, using my actual measurements. I think the two fittings are about 2" apart, given the same measurements. For me, choosing between them was like Goldilocks picking poridge. One was too big, the other too tight. So you might want to order "form" and increase your true chest and waist measurement slightly (1"). If I had done this to start, I would have ended up approximately where I am now. I am thrilled with the final result. After your first shirt, Ricky will probably want you to tell him on your next order by how much to reduce/increase each measurement that needs tweaking. He preferred that to me switching back and forth between "form" and "medium." You must account for shrinkage, especially in the neck measurement. I found that an extra .5 to .75" is ideal after 2 or 3 washings. Of my 10 shirts, Plain Cotton #1 (which I like better than the Sea Island) shrinks the most, and you may want to specify this fabric in particular with a slightly longer arm, say .25". Whatever you do, make sure that you specify a long engough sleeve measurement. The cuffs will fit properly so they won't fall over your hand. Err on the side of slightly too long.