Hi Mike, by loading test dummies, it will be easy to accomplish your task by starting long, and going shorter? If you want to first establish your headspace, you can do this by using a "GO" headspace gauge, or, a shade tree way is to glue or tape shims on the back of an empty and measue with a caliper. you'll find the "GO" gauge measurement allows about .010" (over the length of most all factory brass) in chamber length to accomidate any variances in the brass. For OAL, start by loading an enert dummy / test round at about 1.170"OAL and see if the breechblock closes? It should just snap shut we a little light pressure? Lengthen until it can't, or shorten until it can... Good luck, til...lat'r....GT....