You would never multiply the cosine of the HTAs times the frame size. That makes no sense at all. To Figure the change in reach due to a difference in the HTA, you take the cosine of the HTAs and multiply by the distance from the TT/ST interesection point to the top of the stem. That length is very short in comparison to the frame size and that's why the HTA has little effect on fit.

When you mulitply the c-c frame size (not always the same as the ST length) times the cosine of the STA, all you've done is calculate the frame setback or the horizontal distance between a vertical line through the BB and the TT/ST intersection point.

What you really want to know, with regard to fit, is the frame reach, which is the distance from a vertical line through the BB center to the HT/TT intersection point. That value is the TT length minus the frame setback. Once again, you can't directly compare frames that are different in size.

Here's a link to some info from Cervelo, the orginators of the reach and stack concept. They use a different reference point - the top-center of the head tube. Even with this definition, you can't directly compare the reach of frames with a different stack height.

http://www.cervelo.com/en_us/bikes/2011/R3/geometry/