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Does the engine HAVE to be at top dead center to adjust the valve/rockers? Or can the engine be anywhere in the cycle as long as its a point where both valves are fully closed?

TDC on the compression stroke is the only place both valves are fully closed at the same time. Briggs says piston 1/4 inch past TDC.

But for ease of operation you can adjust the valves having one valve open and adjust the closed one, and then turn the engine over until the adjusted valve is open and then adjust the newly closed one. Which is basically how multi cylinder engines are adjusted.

I always adjust Phillips when the one valve is fully open end of the other is closed and loose. Adjust the loose one, then turn over and adjust the others. When you're all done, crank the engine a few times, and check the gap again, this "breaks in the gap". Adjust again if needed. Pro tip: I gap my valves a little bit tighter for power gains .