I just obtained a (free $$) road bike. It’s a TREK 1200 (not sure of the year). It has a sloping top tube. I want to be sure of the frame size, but not sure how to measure bikes with sloping TTs (most of my bikes are old with horizontal TTs). In looking for ways to measure, I’m seeing the phrase “effective top tube,” but not a complete explanation. Would that be a level (imaginary) line from the top of the head tube to the seat post? (Then measure from that point on the seat tube down to the center of the BB?) — Dan