Originally Posted by
dalava
I think people use "reach" in two different ways: there is the "reach" in stack-and-reach, and then there is "reach" as the total length from saddle to bar/hoods. You are using the "reach" in the former way, while others are using it in the latter way.
Back to the topic of if OP should start with smaller frame or larger frame, I used an example of Cervelo S3 (which actually has an endurance-like geometry). Going form size 56cm to 58cm in that frame, the reach - as in reach and stack, increased by mere 9mm, while stack increased by a whopping 25mm. So if the OP were considering the Cervelo S3, I would definitely say to start with the 58cm. Taking 10mm off the stem is easily done, but increasing stack by 25cm would require all kinds stem-flipping with a big angled stem.
No, reach is not affected in either sense by raising the saddle to get the same fore-aft position on one frame as on another, seat tube angles being equal. You aren't visualizing this whole thing correctly. Actually the smaller frae would have a shortertop tube and reach. Even at proper height of the saddle the distance to the bars would be less on the shorter frame. The high saddle wouldn't change that.
And yes stem flipping would be necessary to overcome a 25 mm difference in head tube height, BUT NOT WITH A HIGH ANGLE STEM. Only a 6 or 7 defree stem would be required. That angle give a 1 in h difference from pointing one direction to the other. Do the calculation online. You will see. I would rather have the flipped up 7 degree stem than the shorter one.