This seems to be a personal preference thing vs an industry standard. We chose to implement it with the front height dimensions versus the opening dimensions because most cabinet shops order drawer fronts and boxes from a third party and often order standard sizes. Either way, CabWriter does the arithmetic, the user doesn’t need to, though there are a few tricks that help selection go faster. I should create a video of that.
That said, it may make sense to give the user an option with a default. No promises, but let me look at it.