I’ve had really good luck with the following recipe:
1) Ask the bowyer what they recommend for their bows. Ron LaClair has always suggested using 10-15 pounds of spine over bow weight at 28” draw for my Shrew bows. Steve Turay has recommended arrows spined right at bow weight for my Shelton and Ramer string follow longbows. I believe centershot and string material as well as amount of reflex play into their recommendations.
2) For every inch above 28” of DRAW length add 5# of spine or subtract if you draw less than 28”. I don’t like leaving my arrows long, so they’re cut one inch longer than my 28” draw. Therefore I have no formula or suggestions if you like it that way, but it will weaken the arrow and cause a need for more spine.
3) For every 5 grains of point weight over 125 grains, add 1# of spine. Thus if you want to shoot a 160 grain point, add 7# of spine.
Those are the GENERAL rules I’ve used over the years with very good success. If your shooting a heavy shaft material like ash versus cedar, you may have to go up just a few pounds in spine. Test kit is great idea. I have three arrows in each spine group I normally use fletched and ready that I just put my intended point weight on and test to see what is flying best. My final test is to step back thirty yards and my broadhead and field point should be grouping together. Good luck. I like Wapiti cedar shafts and Surewood douglas fir. For a real treat I use Paul Jalon (Elite Arrows) crafted arrows. I’ve never shot a finer wood arrow than what he produces.