Spark advance
 

So I rebuilt my 89 N/A a while back the head was torqued wrong due to all-data listing for spec so the head warped and I had to get it machined again. Ever since the rebuild it was hard to get the spark advance just right and it became more difficult after the head was machined the second time. I don't know exactly what was taken off but someone had told me once a head is Machined the spark advance becomes a whe lot more precise to set perfectly I was wondering if anyone knew the best way to set it to the exact potion and yes I have replaced the rotor button and cap spark plugs and wires I'm running NGK laser platinum twin tip plugs now. Any help would be appreciated!! Thanks.

Shaving the head does slightly decrease the distance between the cam gears and tge crank gear, which i believe could make it difficult to set your camshafts perfectly with the crankshact (due to the set length and number of teeth on the timing belt). If the underlying cause of your problem is the camshafts (specifically the exhaust cam) not lined up correctly with the crankshaft, a set of adjustable cam gears can fix that problem.

I do not know how much material needs to be removed to cause the gears to not line up.

It takes a lot to offset the ignition advance any significant amount. I doubt that even if the head were shaved to Toyota's "max allowable" that you'd be more than a degree off. The mechanical timing of the valves would be offset but again not to any level that's going to cause any issues whatsoever.