If you have the head off you can get some plastacine or blu tac and place it on the top off the piston, do a trail assembly and slowly rotate the engine by hand 720 deg. the valves wil contact blu tac nd you can then measure the clearance to the top of your piston. This method works very well and you can see the result. If while you are rotating the engine it starts top lock up stop and check the clearance. Just remember do it slowly and by hand.
Good luck
steve

nah shes all good now, turns out we were following instructions for a earlier modle set up (f**k CROW!!) basicly we just slapped her on and did a trial and error adjustment by 2 degrees each time.