hmmm , for me in MY opinion I feel a little / slight compression of the pad to the cooler /vrm/ what ever . so for me if a 1.5mm gap maybe a 1.8mm / 2.0mm pad heat will expand /warp what ever so if its a loose fit it may get looser or the fit gap may grow as the card heats up and cools down
now how true are the pads you order going to be at that 1.5mm tolerance ? that comes down to experience on what is needed you want the best snug contact with out going overboard on that .
like with the feeler gauge if it slides in with little effort may be a tad too narrow if it was hard to slide in the gap like you had to raise up the cooler a bit that's close to whats needed . [again that comes with experience using the tool ]
but like I say too thick can be as bad as not thick enough . for me being a millwright by trade I kinda got a feel on things as that / expreance.
I was looking for a article or a forum that showed all this but now I need it for you I cant find it the guy showed how he used the feeler guage and all to determine the pads he needed cause the factory sized one were not right anymore on the example card he used in the how to thread thing .
see from here how thing can be ?? [scot w about 1/2 way down ]
''It is odd that some people have "gaps" and others don't with the same model card. I can only conclude that there is some variation in the parts.... Either different thickness thermal pads, or different thickness VRAMs, or different length standoffs resulting in a "mixed" part supply at the factories, so some cards are fine and others have gaps.''
https://forums.evga.com/gaps-between-vram-chips-and-the-stock-thermal-pads-on-the-midplate-Page-3-m2570619-p3.aspx if nothing else order the 1.5mm and some a tad thicker just incase there needed check fit with each used and go with the best fit ??