OK I had a stab at putting your numbers into my excel tool, but I need to caveat all of this by saying I’m not a bike fitter and my entire experience has been about tracking my own fit changes and purchase options. One of my bike fitters said my tool is good and I managed to predict my Retul fit spec for my Tarmac exactly, plus I’ve used it to help spec a custom titanium frame that I’m happy with, but another fitter who is a more advanced research scientist type said there’s an issue with my calculations. I suspect it’s just that I’ve done the angles like a carpenter rather than a scientist and so have sometimes gone the opposite way round a circle than is the accepted norm, but I don’t know for sure. I’m also no excel expert so had to look up how to write most of the trig formulas and I can’t even make total sense of them looking back now. But the tool works for my bikes so might work for yours too.
Someone like @mcneese.chad is maybe better positioned to comment here and I note he often links to Bike Insights which can be helpful.
Also the website I mentioned that allows you to include stem, spacers, bar reach, etc. is Bike Geo Calc and I’d trust it over my spreadsheet if in doubt. It’s a bit confusing at first but is a really robust tool. You need to click on the different buttons at the bottom to show/hide different measurements, then you can click each measurement itself to change the number. That’s just the default bike so you’d need to start by copying over your bike’s geometry chart before you can start playing around with it.
My spreadsheet with your numbers in it as best I can tell is here. The forum won’t let me upload here so that’s a link to my google drive and I’ve made the file publicly available. Should be read-only so you’ll need to download a copy to save changes. Some things worth noting:
- This was written first and foremost to see what it would do if I changed stems and spacers around on a bike I already owned - that’s all the variables in the red/orangey section. I found the green section with absolute measurements difficult to interpret at a glance so I added the yellow section which just shows the difference compared to the bike listed at the top.
- BMC’s manual seems to indicate stack and reach goes to the top of the mandatory headset cap, but the drawing here shows that cap being excluded from the measurements. I’ve guessed that it is 20mm tall which is about average for a chunky profiled one. Double check and edit for yourself as required.
- “Stem Stack” is the height of the stem on the steerer tube - the calculation takes the midpoint of this measurement in order to find the location of the middle of the handlebar clamp rather than the bottom of it. 42mm is whatever Ritchey stem I was using when I first made this - change it to whatever is right for you.
- I don’t know if you’re running any spacers on the BMC or the Diverge so I’ve said ‘0’. Adjust as required.
- I found this post suggesting that the Diverge geo chart goes to the top of the small futureshock cover - if you have the large one on or additional spacers you’ll need to adjust.
- There’s no place to account for your hover bar here, but as you say, that should be straighforward because it raises the stack completely vertically so won’t affect reach with complicated angles.
- I don’t think the Saddle to Bar Drop section is particularly useful here. I added it when building up a fixed gear bike with a different crank length and therefore different saddle height and was trying to understand that aspect of it. Was easier to leave it than delete it though and can sometimes be interesting to know.
- There’s no place to account for different shaped bars or shifters here. I think 12s Shimano road is a few mm longer than GRX800 but I can’t remember how much - 3-5mm? No idea how Rival compares either. The BMC and Diverge both have 70mm reach bars but that doesn’t mean much if the shifters aren’t in the exact same place and bars rotated to exactly the angle the manufacturer spec’ed, etc. as I mentioned before.
- I didn’t look up the height of the mandatory headset cap on any of your prospective purchases. That number is a guess. Some searching online will probably help you find a more realistic number.
- Once you get the numbers right for your existing bikes you can start to see what kind of stem/spacer setup will be required to get similar handlebar XY on the other bikes. Of course this also assumes you know what fit you’re trying to achieve - if I were you I would be trying out a few different stem and spacer combos on the Diverge and taking notes about how each setup feels (far right of the spreadsheet) before committing to a new frame. I’ve got versions of this saved with 10+ different lines including the exact same bike but with different stem and spacer configurations so I can keep notes on my experiences with each and see how they compare to each other.
In terms of seatpost choice, I’ve always just gone to this triangle calculator and put in the difference between seatpost angles as ‘Angle C’ plus my seatpost height for both ‘Side a’ and ‘Side b’. That tells you the distance between the middle of identical offset seatpost clamps at two different angles. In your case the BMC will be the ‘steepest’ or closest to 90 degrees, meaning it will be the furthest forward. All the smaller numbers (or slacker seatposts) on the other bikes will be further back compared to the BMC, with the triangle calculator telling you how far.
Then a setback seatpost will be Xmm further back again, so in the case of the BMC versus the Diverge you’re going from 15mm setback to 20mm setback, which means the centre of the clamp on the Diverge should be the distance shown in the triangle calculator PLUS 5mm to account for the extra setback over the BMC. (I think that’s 9.5mm for the angle plus 5mm setback so the clamp should be 14.5mm further back on the rails on your Diverge compared to your BMC to achieve the same saddle position relative to BB - or to phrase it the other way round the saddle on the Diverge has to be slid 14.5mm further forward in the clamp compared to the BMC to get the same effective position.)
I hope that helps. If not, hopefully a real bike fitter will be along soon to tell you all of that in a much simpler and clearer way which is more relevant to your questions!