There’s a bit to it. Add 5# spine for every inch of arrow over 28″ AND 5# for every additional inch you actually draw past 28″ (some guys make 30″ arrows but only draw 28″). Reduce for every inch under 28″ And add 5# if you shoot a head heavier than 145 grains.
One good bet is to order a “try set” of arrows in different spine increments. Some fletchers even loan a set to you with a security deposit refunded when you send them back. Three Rivers sells a try set. That way you can try several and see what works. Release, bow shape and type all make for variances in needed spine.
Recurves, in general, are more forgiving of a heavy spine; so round up rather than down. Longbows (not cut to center shot) want the proper spine .