Oh and when it comes time to figure out how much seatpost setback you need that’s another mix of trig and guesswork. Assuming you have a saddle position you want to replicate and it’s currently clamped square in the middle of the clamping area on the saddle rails, then any bike with the same seatpost angle will require the same seatpost setback to end up clamping in roughly the same spot.
If the angle differs then it’s about solving for an isosceles triangle that has two legs at your saddle height and an acute angle that is the difference in the seat tube angles between the two bikes. Assuming your stated saddle height of 780mm that’s going to be a difference of 6.8mm for every 0.5 degrees of seat tube angle change.
So from the Diverge to the Otso you’re going 1 degree slacker which would put the centre of the clamping area 13.6mm further back compared to the BB if you used a seatpost with the same amount of setback. If you’ve got 13.6mm of useable clamping space behind your current saddle clamp and in front of the limit mark on the saddle rails then you can use the same seatpost setback on the Otso.
I’m pretty sure I’ve oversimplified that a bit because it’s arcs and not triangles so that calculation isn’t strictly accurate, but it should be close enough to work out which seatpost setback you’re going to need, especially when the differences between angles are within a degree or two.