When I list a bike, I always tell how I measured it. IE. Center of crank to top of seat tube.

I figure that at least lets me off the hook for those that think of sizing differently. Old bikes had several different places they'd measure to depending on the whim of the manufacturer. Commonly the top of the seat tube, but some measured along the seat tube to the extended centerline of the top tube or the extended top of the top tube. For most bikes back then that wasn't but 1 to 1.25 cm difference anyway you measured.

My 78 Raleigh Competition is supposed to be 59cm as the previous owner told me. It measures 59 cm to both the top of the seat tube and the top of the extended line of the top of the top tube.