My machine dates from 1990 making it...crap...23 years old! Thus it's wattage is still in the 3 digit range. If I simply place the bag upon the turn-table, barely half of the kernals pop. But I elevate the bag by using a "Corning" brand "grabber" (a squat cylindrical soup bowl with a flange for, well, gripping) topped with microwave safe plate. this positions the bag closer to the dead center of the chamber where the "emitter" is focused. Then I set the machine for 3 minutes 33 seconds and just keep an ear and nose open when it had reached the last 33 seconds.
Also, I don't cook the standard sized bags as that is usually to much for just me (and I'll "graze" until it's all gone, can't stop myself). So I purchase smaller, "single serving" bags that are just right and so I don't feel "bloated" afterwards.
P.S. I guess I should state that, no, my microwave does NOT have a "popcorn" button given its age.