Scot & All-
While there is no special bicycle metric to imperial conversion formula, there is some differences as to the points of which are measured.
Decades ago, frame size was commonly measures center to top, for the seat tube for example, Masi measured to the top of the seat lug, a wee bit different, later (in the mid 80's, possibly earlier) one had to ask the distributor where they measured, as center to center became common, to the mid point intersection of the top tube to the center of the BB. Remember this is for 1" top tubes, not very many oversize prior to the list cut off. Dimensions creep as you read.
There are also differences as to how a chainstay is measured with a horizontal end. At the front as often assembled? At the mid point? At the intersection of the stays, a virtual point.
Bottom bracket drop or height? Depends on the wheels. Wheel diameter, also varies a wee bit, even for a given size.
Getting softer? I tried as a test a while back to see if quoted geometry that was offered by some was correct, (had more time) Most of the time the numbers provided were off when applied to a CAD layout, or there are some bikes out there that have tubes that don't intersect or forks that would look quite unusual at the minimum.
Its a soft science.
John Jorgensen
Palos Verdes Ca