Seatpost/stem sizing on new bike

[dexterdent]

I'm pretty far down the rabbit whole of getting myself a new road bike: https://www.evolvebicycles.com/product-category/frame/

The frame size is pretty easy to choose, but I need to make sure I choose the correct stem/seatpost. My current roadbike does not have nice published geometry data (I have measured it, but lower confidence), but my gravel bike, on which I have also done a bikefit and am very happy with, does. So if I compare the two I get this:

https://bikeinsights.com/compare?geometries=67ce8e5aaa8c97001b264019,5fa21a950cb1bf00179dcf19,

The similar reach implies to me that I should get the same stem (i.e. 80mm)- it's a one piece so I have to get this right first time. I suppose there could be slight differences in the handlebar shape too which might impact this.

The harder part for me is the seatpost choice:

 

Any advice on how to choose this? My Rondo seatpost has about 170mm of exposed post from the clamp to the rails, and the seat is clamped mostly to the front part of the rails (i.e. the seat is relatively further back from the handlebars). It's a Easton EC90 20mm offset post.

I'm not asking for anyone to do the work for me - mostly interested in how to figure this out.

 

[droo]

A level, a wall and a tape measure will go a long way.

Level for reference against wall and floor for datum (cos walls and floors are never reliably straight), and a tape for the rest.