[jack_jennings]

Except that isn't how font metrics work—the size of the type on the body is not necessarily equivalent between two different fonts. There is no stanard that says that 12pt Times shares any vertical metrics with 12pt Helvetica, or 12pt anything else.

The most informed comparision can be made when the x-height is optically equivalent. The other option, cap height, isn't as useful because it is then harder to judge the lowercase (a majority of the letterforms).