I promise that's not always true.
In fact, about 2 weeks ago I had it actually happen to me for the first time ever... the clamp-on block on my 308 moved from where I had it set back against the shoulder, and it quit working. I put 0.30 gap back in and it worked again (ive since been reading that company supplies junk screws that stretch w/ their block)
~2x the diameter is only true in certain cases, eg carbines w/ .060-.065-ish ports, you might get away w/ it there depending on how well everything is cut. I recall somewhere around .090+ on a rifle port to into ~.120 on most blocks, i.e. def not 2x. going from memory so can't recall exactly. but that says you'd get 0.015+ overlap when pushed back...
plus pre-dimpled barrels usually assume the ports directly lined up to center the set screw, so you get best contact w/ the gap, regardless of alignment.
for the OP, all this is into the minutia. push it back or leave a gap, whatever... try it and see, or call the manufacturer and see what they say.
-rvb
I said almost twice the size. I can only speak from my experience. I've put about 15 ARs together and troubleshot a few others for friends with gas related issues. I have yet to see any overlapping with the block against the shoulder. But my sample size is only ~18. So sure it's possible I suppose. With folks making barrels with oversized ports you could ever so slightly overlap and still have enough gas to cycle the gun reliably. Of all the ones I've troubleshot they were Mid-length and mic'd .075-.078. And when I removed the gas block I could see the burn ring. Block was against shoulder and top of burn ring closer to barrel port but not overlapping. In each case the gas block was a no name leaky piece of junk.