For a LTCO I will sometimes go up a needle size, especially when I need to be sure it's not tight. I found that in trying to get the stitches to look nice I was tightening them too much. A gentle pull on the yarn over my thumb helps get them snugged but not too tight. A knitted or cable cast on makes it easier for me to maintain a looser but still even tension. HTH and of course YMMV.
I should never overestimate my basic arithmetic skills.