Halyard attachment point to gaff

I have two split wooden gaffs and one new one. I need obviously to attach the halyard attachment loop but it's in slightly different places on each of the old ones. What is the best way to work out the ideal position? I'm wondering if I lay the rig down on the grass and work it out that way, but I don't think I want the knot in the halyard going as far as the masthead, surely I want it just short to retain some uphaul flexibility? And what to people use to attach to the loop - a knot? A shackle? I think it should ideally sit to the side of the gaff so the gaff goes as vertical as possible, but can't work out how that would happen given the loop sits between the gaff and the mast. Thank you!