Measured a weird opening three times, then my installer caught my mistake
I was prepping a kitchen pass-through in an old house in Pittsburgh and kept getting 74.5 inches no matter how I squared it up. Turns out the floor was sloping a half inch across the span, so my cabinet guy suggested I measure from the high corner only and level the base. He was right, the uppers finally line up flush. Anyone deal with hidden slope like that in their own reno?
Measuring from the high corner only works if you're leveling the whole base first. If you just take that 74.5 from the high side and set the uppers without shimming the low side up to match, your boxes will be sitting level but your gap at the top or bottom will be all over the place on the other side. The real fix is finding the high point, then building up the low spots with shims or a sleeper so the base plane is actually flat. A half inch across a pass-through is a lot, that's not something you can just fudge with trim. Old Pittsburgh houses are the worst for this too, nothing in them is square or level. Bet that floor has been sagging since like 1920.