You can, but going by the AMO length is hit or miss. The old "rule of thumb" is 3" shorter for a longbow, 4" shorter for a recurve, and that works for a lot of bows--but it isn't AMO. AMO is as Jason described. Some string makers don't know that. Add to the mix that a lot of bowyers don't follow AMO specs when making/marking their bows, and it can be a mess.
The safe bet is to order the string by actual string length--that's how I take my orders. Even if a bow is built to AMO specs, there's a range where you like the bow tuned and it might be at the high or low end, which can make a lot of difference in the string length.
Yep, adjusting a string .5" isn't a big deal, but you shouldn't have to adjust it 1.5".
Chad