That's what I usually do also - a string a couple inches shorter than the bow. Don't know if you've learned to make your own bowstrings yet? If not - it's worth learning straight away. It's cheap to order some Dacron and you can make the jig easy. Like Josh said you can adjust the length by just twisting or untwisting and it helps during the tillering process.
Once you start removing wood mid-limb it'll go quick! Looking forward to seeing your first, it'll be far and away above mine!