On a bow of that age, the string may have stretched as much as 1 1/2" or more, if it isn't a newer string material. I would attempt to get ahold of the manufacturer. This info is written down somewhere. If the ata and brace height are real close to specs, and the draw weight pulls to what you expect, you can assume the string isn't too far off and taking measurements will probably work okay, but I'd still prefer to get the actual correct length for the bow.