Like SE-Bmxers said: For TOP TUBE you would normaly measure along the top tube, from the center of the headtube to the center of the seat tube.
BUT: The problem is that if your frames have different seat angles, AND/OR, if the top tubes are different heights (Modern frames have very low top tubes that are at more of an angle than an older frames) you will get different measurements for what might actually be closer sized frames.
Can ... open ... worms ... everywhere