This should be an easy question to answer. It is not and here's why:
Mass-manufactured strings are labelled by the bow length, not the actual string length. So a string is labelled as 68" when it's meant for a 68" bow. The length of the string is not actaully 68". This does not allow for the fine tuning a custom built string provides. Typically a string for that length bow should be about 65".*
String length should be measured under 100lbs of tension according to AMO. That's great, but who has the abililty to replicate that setup at home to measure the length of the string they need?
Manufacturer string length recommendations vary. Hoyt's string length recommendations are about 1/4" longer than other manufacturers. And Uukha's recommendations vary based on the limb curvature and generation of the limbs.
Your best brace height for a setup may vary. If you spend time tuning your setup and determine your best brace height, you may have to add or remove quite a few twists to get there. That's no problem but it may not result in an optimum number of twists in the string.
So how can you overcome all this and know what length string your bow actually needs?
Know your equipment and look up the manufacturer recommendation for bow and string lengths. Share this information with your string builder to double check.
2. Customize whenever possible. If you get a new bow and you have an old setup of the same length, start tuning with your old string. Then once you have a good brace height that works well then order a new string for the new bow of that length. If you don't have a previous string that is close to that length then the manufacturer's length recommendation is the best place to start on a new bow.
3. Measure your string using this method: https://www.ocdstrings.com/post/how-do-i-measure-my-string-length Take that measurement and have it built to that exact length so you don't end up with ropiness from too many twists or ballooning from to few twists.
Hope this helps the next time you are ordering a custom string!
* The assumption that a string should be a about 3" less than the overall (AMO) bow length is assuming a modern bow, string materials, and building methods. Dacron string material or flemish twist construction are a different story.