I heat treat my bows when I have them tillered out to 20" or so. I have also done it when the bow was completely tillered. Hackberry needs to be heat treated or it will take some set. Ive only done it once without a heat gun, I reverse braced it and heated it over hot coals from a campfire. I like using a heat gun better, i feel like I have better control. That hackberry sapling should make a fine bow, be sure to post some pics. I love hackberry