I hope you don’t mind me reaching out to you for a little more advice.
I have the identical saddle on both of my bikes. I measured the saddle set back on my BMC and Diverge- from the saddle tip to the center of the BB is 7 cm on both bikes.
I also measured from the tip of the saddle to the center of the handle bars and that measures 55 cm on both bikes.
The BMC has an 11 cm -12.5 degree stem and the Diverge has a 9 cm +6 degree stem.
The stack on the BMC to the top of the stem is 696 and to the top of the stem on the Diverge is 650. On the Diverge with the stem turned up I believe that adds about 2 cm of stack and the bike has a Specialized Hover bar which raises if another 1.5 cm- so total stack on the Diverge would be 685 (and I lose stack on the BMC with the stem inverted at -12.5 degrees, so I am assuming they are close).
I am not accounting for the changes in reach that the additional stack produces, but it would seem that I’ve done this with saddle position as both bikes have identical saddle set back and identical distances from the saddle tip to the center of the handlebars. The BMC has Di2 12 speed and the Diverge GRX 800 shifters, while the bikes I am considering are SRAM Rival. I am sure there is a difference in the geometry of the handle bars, but that might be too much information.
The part I am confused on is how to apply the above dimensions to the bikes I am considering. Would you be able to provide any additional insight? The one area I think would be most critical would be how to achieve the appropriate saddle set back (7 cm), as this will determine overall reach and stem length. Would one of the bicycles geometry be better than another to get the dimensions correct? Would one bike definitely not work?