My advise is to get off that long string as soon as possible. Or as soon as you can brace it to 3-5".

If you are at 18lbs at 13" of string travel, you really need to get on a short string. If not, you might end up with a messy bow and not enough wood to fix the problems with. Long string tiller is the equal to floor tillering. It wont tell you much. I can't see much until I get to a brace. I usually start going on a short string, with about a 1"-3" brace once I get about 10" of string travel at about 10lbs less peak weight. I shorten the string as I move along. If im going for 60 lbs. Ill start shortening my string when I get around 50 lbs at 10". As I soften the limbs, I will twist my string a little each time until Im at about a 4" brace. Once I get to a decent brace, I can see a whole lot more about what is going on.