I don't know that the exact percent chance has ever been posted, at least I've never seen it, but it's all dependent on RL luck and the RNG. I spent a week in Heartwood, about 6 hours a day when ML first came out (was lucky enough to have been on vacation that week) and went from 50-GM Bowcrafting. In that time, I got 2 Oak and 2 Ash runics. I've heard of people going there and getting a Heartwood after their first hour, then others that wouldn't even know what a runic looked like if they hadn't seen one on a vendor for 15 mil. So....ya feelin' lucky?