You really don't need to worry about those measurements. Just follow the RCBS instruction sheet for setting up the sizing die and you're good. After sizing, check length, and if over 1.760" trim back to 1.750", chamfer and deburr. Otherwise leave alone.
I was under ther impression that all once fired cases had to be trimmed to 1.750, is this correct or not ? If you don't trim and have cases of various length, what does that do to the accuracy ?