Peter Alexander's book "The Gunsmith of Grenville County" dedicates 10 pages to tuning a lock - both flint and percussion. Essentially he focuses on minimizing friction and wear then modifications to enhance performance such as a perfect fit between pan and pan cover, smooth, safe trigger pull, one-position sear arm, proper heat treating of parts and how to test a lock before you buy it, based on the preceding points. Beg, borrow or steal a copy of the book and photocopy those pages for reference. Every fault and manufacturing short-cut he discusses, I have seen in the new locks I've had.
I've learned that we tend to like the attributes of sett triggers but they can hide a lot of undesirable faults in our locks whereas a well tuned lock with simple hunter's trigger need not give up much to the sett trigger as far as a light, smooth and safe trigger is concerned.
With a new lock, the first thing I do is remove the main and sear springs then check that the ####/hammer can swing freely by it's own weight - it often doesn't. If not then there is undesirable friction in the plate-tumbler-bridal fit and that needs to be reduced ASAP. The inner face of the plate should be perfectly flat from end to end (BEFORE you inlet it into the stock). The lower arm of the mainspring should not rub on the plate. For a safe and smooth trigger release, the line of the full #### notch face should extend though the axle of the tumbler and the sear face should fit flat against the notch face. That's a really quick summary of some of the points he raises and illustrates with some excellent drawings. Go slowly and plan to spend a couple of evenings on the refinements - they are all worthwhile in the long run.