I realize this doesn't help You determine scope cant in any way shape or fashion . Yet it nearly shows the method I use for mounting and lining MY scopes up . Exceptions being I use a laser cartridge bore sight'r ,believing they are slightly more accurate than eyeballing .
I also have a couple of ( 1.006" and 1.1181" )precision machined 4440 hardened scope ring alignment fixture bars . They have a level machined area for My Starrett machine level to rest in . Also a few tube of the same dimensions with a cartridge specific opening in either end . .223 & .308 and 7mm and .338 in both of those tubes . Purpose is laser cartridge case fits each ,so as to make scope and bore measurements as well as alignment much easier . I don't believe I've ever run into a reticle cant ,as when I line everything up and place my scope in their rings I line vertically with target using those laser dot marks . Obviously I want BOTH marks centered onto the targets vertical aka plumb line . As I said it gets Me within an 1.0" of zeroing 95% of the time .
Another thing I do before worrying about zeroing ,is shooting #1 clean bore shoot #2 clean bore through #8 shots . Has worked well for ME .
Unfortunately as 6.5CM wasn't available then ,My machinist didn't make one in that caliber and he's now deceased .