[Dave]

What you're doing makes no sense. What you really want is for someone to calulate the difference between your old +17 stem setup and the new, but you didn't provide nearly enough info to do that. An accurate comparison requires the old and new headtube stack height, and the stem clamp heights in addition to the angles you provided. That's way too much trouble to bother with.

If you want the saddle positioned at the same setback as the old bike (4.8cm), then all you need to do is position the right crankarm horizontal, place some masking tape on the arm and mark 4.8cm behind the crank bolt center. Then place the bike on a level surface (level the TT if it's non-sloping) and use a level or plumb bob to verify that the saddle tip lines up with the mark on the crank arm.

I can't figure out why you put a much longer stem on the new frame. The "reach" of these two frames is almost identical. Here's how it's calculated:

Reach is the TT length minus the frame setback. Setback is the cosine of the STA times the c-c frame size.

Unfortunately, you left out the frame size, but from your previous post, here it is:

Old bike: 58 cm ST c-c, 7 cm BB drop, 75* ST angle, 77.7 cm from c of BB to top of saddle measured along the ST, ideal fore/aft saddle position is 4.8 cm from front tip of saddle to plumb line intersecting the center of the BB.

New bike: 59.5 cm ST c-c, 8 cm BB drop, 73* ST angle, 77.7 cm from c of BB to top of saddle measured along the ST, ideal fore/aft saddle position is ??? from front tip of saddle to plumb line intersecting the center of the BB.

Ideally, both frames should be the same size when making this comparison, but this should be close enough. The first frame has a setback of 58cm x cos75 = 15cm. The reach would be 57.8 - 15 = 42.8cm. To improve accuracy, I’ll calculate the reach of the new frame at the same point along the seat tube. 58 x cos73 = 17cm. The reach of the second frame would be approximately 59 – 17 = 43cm. This tells you that there is a measly 2mm difference in the reach of these two frames.

This analysis ignores the (usually) minor variation that occurs due to differences in the HTA.

The bottom line is, once the saddle is set properly, any difference you measure from the old 53cm saddle tip to center of bars is an increase in the reach due to the new stem.