AMO (ATA) specs are basically 3" shorter, longbow or recurve. It goes something like the AMO bow length is the string length plus 3". 'Course that isn't always the case--safe bet is to order or make a string based on the known string length. I've seen longbows get strings from 1.5" to 4" shorter than the marked bow length, recurves getting strings 2.5" to 6" shorter.
The old rule of thumb is 3" shorter for a longbow, 4" shorter for a recurve, but that is hit-or-miss. I learned that the hard way several years ago.
Chad