Like Seat tube, there are several variations in how you'd specify this, for frame comparisons.
a. Actual Length
b. Horizontal following seat tube angle
c. Horizontal to vertical line above actual HT junction.
I think "b" would be most useful to compare frames. However, some websites like MEC give the horizontal distance to a vertical point above the actual C-C point where the seat tube meets the downsloping head tube. This point is further forward than if you followed the actual seat tube angle.
Since I only have one field, I give whatever the mfg gives. We really rely on Stack and Reach for frame size comparisons.