help, somebody gauge question

So, I am going to knit a sweater! Yes, the February Lady Sweater. A little help please as I've never been down this road before.

ok, so if i knit tighter than normal, i am at 4 1/8" per 18 stitches (pretty darn close, i just have to remember to knit tight, which is not normal for me...)

so, that means that for every 18 stitches, when i cast on 83 (xs size as everyone says it is big), by the time i am finished i will really be knitting the size that has...

let me do the math

18 stitches goes into 83 approx 4.6 times..so i will be adding an extra 1/2 inch? which turns out to be 2 stitches? so that would be 85.. and there is no 85, only 3 sizes that call for 83, you only change the amount of increases etc....maybe?

does this even make sense??

so am i good to go?

aaaccckkkk back to socks where i don't care about gauge, the socks go to the person they fit :)