In brief and to over-generalize...
It can depend upon one's body "type" (body morphology). Meaning if one is shaped like a "ruler," things may be different than if one is shaped like a pear, etc., etc.
IMHO, as shared, the concept of "waist" seems to have changed. CW Period "high" waisted trousers worn at, above, or around the "navel" line are different than modern "hip huggers" worn down below the hips. (IMHO, for things "Confederate," higher waisted trousers make it possible to wear "shell jackets" without a draft....
However, some vendors offer trousers that are cut more along the lines of modern jeans and some lads wear them skin tight down below the hips in the Butt Crack Gusset Zone.
So, IMHO, measure around the "naval."
However, not all makers seem to use the same "sizing." For me personally I can wear one vendor's Size 34 (Size 2) but have to go to a Size 36 (Size 3) from another vendor. Or, another vendor's Size 3 fits snug, while another's Size 3 is loose.
However, when looking at original trousers, there are the Quartermaster specs, and then there are variations in the actual dimensions.
Again, being brief and over-generalizing...
In gleichem Schritt und Tritt, Curt Schmidt
Not a real Civil War reenactor, I only portray one on boards and fora.
I do not portray a Civil War soldier, I merely interpret one.