Road Geometry Versus Gravel Geo

[fignon's barber]

Over the last few years, the geometry of gravel bikes has drifted away from that of a road bike. Longer top tubes, super short stems. It's getting more difficult to match your road fit to a gravel bike. For example, my road fit is a top tube of 555mm, with reach between 385 or 390 with a 110 or 120 stem based on reach. That puts me squarely in a size medium for most brands.
I was just looking at the geometry chart of a popular gravel bike, and a medium size came with a 411mm reach and a short 70mm stem.
Is there a formula for matching road/gravel? Can you simply add reach and stem length to compare the two, or am I missing something?