I've done it with a dial indicator before and it was pretty close, any differences I'd blame on deflection, pushrod length, operator error etc..If you really want to do it this way, then the better way would be to pull the intake and use a solid lifter and multiply that x your rocker ratio. But if you think it's an aftermarket cam you could pull the timing cover and if it is it will give you the manufacturer and part #. Or better yet call your favorite grinder and pick a cam you really like and put it in.