I don't think you understand the procedure for using a piston stop to find TDC. You will be be making two temporary marks on the PMA, one slightly before and one slightly after TDC. TDC is then located by finding the spot exactly half way between those two temporary marks.
Start by roughly locating TDC. Look through the spark plug hole and watch for the piston to come up to it's highest point. Insert a rod of some sort into the plug hole and rest it on the top of the piston. A drinking straw works well. Bump the piston back and forth slightly while watching the straw rise and fall to verify approximate TDC. After you locate it as best you can, turn the engine backwards (CW rotation direction) and lower the piston about 1/2". Install your piston stop so it touches the piston. Bump the piston back and forth a couple times to insure it is making good, solid contact with the stop. Now make a temporary mark on the PMA aligned with a temporary reference mark on the side cover. It would be best to make these marks off to the side as your permanent marks are going to be at the bottom. The front mounting tab for the round cover would work well. You have just established your temporary "before" TDC mark .....
Now, lower the piston away from the stop (CW rotation), bringing it all the way down and back up again until it hits the stop again. Bump it up against the stop a couple times to insure good, accurate contact. Now make a second temporary mark on the PMA. You have just established your "after" TDC temporary mark .....
True TDC will be exactly half way between the two temporary marks you have just established. Remove your piston stop and find that spot. Now you can make the permanent PMA mark and a permanent reference mark on the side cover at the bottom .....
But, you have only established one out of three reference marks that you need, TDC. You still need reference marks for idle (retarded) and full advanced ignition timing. They are 15° and 40° BTDC so a degree wheel would be needed to accurately find them. The simplest solution would be to acquire the timing sticker shown above from Hugh's Handbuilt. Apparently he'll send you one for nothing if you send him a stamped, self addressed envelop. Then marking the PMA for TDC is all you would need to do.
