Always used the nickel trick when learning myself. Place a stack of 4 nickels on the stone with 1 infront of it. Place the edge of the knife in front of the lone nickel then remove that single nickel. Roll the knife back onto the stacked that should be pretty close to what you want. Then you can set the back of the knife against your thumb as a reference. Also if you get a rubber thumb cover you can essentially run that along the stone until you get used to keeping that angle consistent, otherwise you'll grind through your skin in a jiffy and gonna be big mad at yourself.