Help me confirm my measurements and back spacing thought

Are you talking steel, or aluminum wheels?
Quite often aluminum wheels have wider outside measurements 'cause the edge of the rim is thicker.
Won't affect the section width, 'cause the rim width is the same but may confuse you when comparing back spacing of two different wheels. Given the choice always get the bigger back spacing, as others have said.
Remember measure,measure,then measure again!
And don't forget to bring your own straight edge(s) and tape(s)!