How do I determine the string length for a recurve if AMO length is unknown?

Take it to a pro shop

On my old recurves the string length is equal to the distance measured tip to tip of the unstrung bow. in other words when unstrung the string is held on by the nocks on each tip with just enough pressure to hold the string in place. maybe if you know the fistmele you can bend the bow parallel to a wall while a friend measures the fistemele and string length or like brice says go to a pro shop. Hope this helps some.

measure along the curves of the bow. If you cant figure it out, start low and keep twisting it up.

Good question. Rule of thumb I use is to measure nock to nock along the belly of the bow and then subtract 4" from that measurement to have the string length. Heard this years ago and for the few recurves I do it seems to work. Was going to post this question on the traditional forum a while back to see any new ways.