Does this all make sense vs what you meant?
Yup. You got it almost exactly. Just one thing...
I have had to assume a linear relationship...
In my experience, the relationship is not usually linear. Generally, it's more logarithmic. (i.e. maximum aperture usually drops off more quickly as you start to zoom in from the widest focal length and then levels off as you reach the longer end.) Unfortunately, I don't know of a formula that works for all lenses. But if it helps, my RX100 (original) goes from f2.8 to f3.2 at around 40mm (full frame equivalent, 15mm actual, 1.5X zoom power).
P.S. re: "a GM1 with kit lens cannot match f2.8 at all" -- true, with the kit lens
, as you said. But the strength of an interchangeable lens camera is that there are other m4/3 lenses that can match or even exceed f2.8