I'm not sure either seat tube height c-c or c-t are important, for me at least. What is really important is being able to create the correct relationships: BB center to saddle in the vertical and horizontal directions, and BB to handlebar relationships in the vertical and horizontal as well. A good frame design is one that allows me to create those relationships with available parts. A frame can stop me if: the seat tube is too steep (can't reasonably get the saddle back far enough), the head tube is too far forward (can't get correct reach with a decent stem length), or the top of the headtube is not in the right place (can't angle or raise a stem up or down enough, with a suitable extension). It comes down to three basic specs: seat tube angle, height from BB to top of the head tube, and horizontal distance from BB to the top of the head tube. Notice top tube alone is not right, nor is seat tube length.

You can get a good cut at your target values by measuring them on a bike that fits well.

For a frame with horizontal top tube, the head tube height requirement comes down to a seat tube length requirement, as does the recommendation for "a fistful of seatpost." The recommendation to "when in doubt go one size larger" is to facilitate a higher head tube top. Top tube length is to ensure adequate BB to head tube horizontal distance, but it's only a valid recommendation if all seat tube angles are the same. Some portion of the top tube is behind the BB and some of it is ahead of it.