For a given wheel diameter, I just look at head tube lengths to zero in on frame sizes.

The OP makes a good point about nonstandard standover measurements. On the other hand, it's worth remembering that the availability of compact frames means that short-legged people as well as people with longer legs can now end up with bikes that fit well.

After all, bikes with horizontal top tubes became common only after bike companies introduced the use of lugs so that they could use semiskilled labor and still speed up their production lines. Horizontal top tubes just meant that the companies didn't need to stock too many different head tube lugs. If you were much shorter than the average male in those days, you'd have to buy a mixte or a ladie's frame or else be very careful about standing over the frame.