I have discovered that most knife handles shouldn't be much more than 3/4 inch thick, but since this is a big knife, I went a bit larger. Set your calipers to half the thickness you want, and start rasping. You could use a plane if you wanted to. Continue the filing until you get the correct thickness on both sides. This is faily quick if you have a sharp rasp, just make sure to file evenly on both sides.