routercnc shared a fantastic spreadsheet in the following thread; it can calculate the sag of some simple geometries, you might have some fun with the more complex profiles but take a look anyway.

http://www.mycncuk.com/threads/2023-...ign-principles

Pros of placing the rails on the same face as Z are that the Z axis is a lot simpler to construct - you only have to have a single plate to connect the Y and Z carriages.

Con is that you increase the distance from your cutting bit to the gantry, which increases the rotational force around the gantry causing more deflection.

JoeHarris' build has a good description of how he joined the profiles in the way you're looking to do by using shims and then drilling and bolting through the extrusions.