This way is easier and requires no math. Take a bolt, brass if you can get one, and thread it in through the spark plug hole. Try not to thread it all of the way against the cylinder wall on the other side of the cylinder. Slowly rotate the engine until you feel the piston hit the bolt. Back the bolt off a little and turn the engine until it gently hits the bolt. Repeat until the engine doesn't hit the bolt again. You have just passed TDC when this happens. Mark the balancer where it lines up with the 0 and you should have yourself a TDC. There is some error in this because the piston remains at TDC for some number of degrees of crank rotation so you can move the crank back and forth a few degrees and the piston will remain at TDC against the bolt. But, it has to be more accurate than trying to relocate the mark 10 degrees.