That link goes over what AMO/ATA says very well, but neglects to mention that a lot of bowyers don't pay any attention to AMO/ATA specs. Going by bow length is a shot in the dark, and something I quit doing years ago. Not only do some bowyers ignore the standards, some archers like their bows tuned outside the specs.
The best way to order a string for your bow is to measure an existing string that puts your bow at the proper brace height, under tension.
I've seen longbows that got strings anywhere from 1.5" to 4" shorter than the "AMO length", and recurves get strings from 2.5" to 6" shorter.
Chad