Measure your dry fly hackle to assure the right proportion. The Griffin Hook and Hackle Gauge is a great selling....and must have fly tying tool. Simply wrap the feather around the post and where the barbules splay out to tells you what size hook the feather is appropriate for.
Also has a hook gauge! Just lay the hook gap alongside the tool and it will tell you what size you have. We sell these as much to folks for their fly vest to determine hook sizes as we do to fly tyers to measure hackles.