I don't recall that it's terribly critical except that excessive play can cause the ignition timing to dither.

However, you must have some clearance -- otherwise the camshaft can seize tight and bind the engine.

End play is controlled by the bronze retainer that's bolted to the block just behind the timing gear. Too much play may be related to a worn retainer, while too little may result from a too-thick retainer.

Be sure you're aware of the steel collar (or spacer) that fits over the camshaft and fits just inside the retainer. That collar provides a shoulder against which the timing gear seats when you torque down the nut. Normal engine gunk and varnish will glue the collar to the cam and even coat it so you don't know it's there.

The thichness of that steel collar (spacer) can also affect end play. Be sure that the collar is properly seated against the cam and that no gunk or grime is artificially spacing it further out.
--
Don Foster (near Cape Cod, MA)