It's a matter of convention, not advocacy. Most Italian builders measured center to center, everyone else center to top. With conventional steel frames you added 1.5 cm to c-c to get c-t.

With sloping top tubes the main fitting criteria are standover (can you straddle it comfortably), virtual top tube length (horizontal between top of head tube to seat post--is the reach right for your torso and arm length), and head tube height (does it put the handlebar at a height where you want to reach it?).

Sizing a sloping top tube bike is usually virtual. It's usually the intersection of the distance between the center of the bottom bracket up the seat tube and seat post with a horizontal drawn from the top of the head tube. Except for LeMonds, which drew the horizontal from the center of the top tube where it met the head tube, and bikes with extended head tubes like the Specialized Roubaix, which draw the horizontal from the top of the top tube where it meets the head tube.

Just eyeball a couple sizes and try 'em on.