I like to use a file as suggested to hold the cam. Also, the IP pulley is spring loaded sort of speak so its tough to get it where you want it. The trick is to put your ratchet on the IP pulley and hold it where you want. Then clamp the belt to the tensioner or the cam pulley (just to keep it taught). That way you can ensure you get the belt nice and tight between the crank and the IP pulley. I've ran into huge issues with slack there causing massive timing problems. It's gotta be tight. There is an exact number of teeth that can be between the pulleys sort of speak.
I hope that helps although from the sounds of it you've already solved the problem.