Yes, a key unit is always 3/4" = 19.05 mm, otherwise aftermarket keycaps would not fit. It is important to not round off to whole millimetres, or you could get compatibility problems near the edges. A keycaps must always be a little smaller than the key's "footprint" on all sides, but that varies a bit and I don't think it matters that much: and it can differ a bit between keys of the same profile in different types of plastics.
If the keyboard case has a top, then there is also added gap around the key footprints in the case design so that spacing between key and case matches spacing that between keys.
Spacing between clusters is
usually 1, 1/2 or 1/4 units. I think Topre and Filco have the same spacing between function-key row and alphanumeric cluster: 1/2 unit. They differ in horizontal spacing though: I think Topre uses 1/2 unit whereas Filco uses 1/4 unit between alphanumeric cluster and nav cluster.
CAD files for Filco-compatible plates should be available. The first ones I saw fit only Filco, but later revisions fit also the CM Storm Quickfire Rapid which had different standoffs inside the case.
Check out
Swill's Plate & Case Builder that can generate CAD files from
Keyboard layout editor's format.
