Many years ago Like 30 or so I purchased the 35-75 Cam from Al Constantino in Florida. He also at the time had a very nice Catalog that was full of experiments and installation tips about Modifing the X1/9, don't ask about the book, I lent it to a friend, now all history.
In the text was stated that the cam was about 1 tooth off and the simple way to check was to put the engine at TDC on Number 4 and now look at the Transmission Bell housing. Use nail polish on at small screw driver and mark the zero and 10 degree marks on the fly wheel, then rotate the engine backward 10 degrees and mark the flywheel again until you have moved the engine 35 degrees backward. The intake valve should now just be touching, using a feeler gauge to prove. If not then rotate the engine 10 more degrees and recheck one way or the other. I found on my camshaft the mark to be out by 1 tooth. I then filed a new mark in the cam pully( slipped the belt a tooth) when I had the 35-75 cam in the engine.
TonyK Grimsby Ont Canada.