Been there, done that! I can't EVER seem to cast on the right number of stitches if it's anything past 30...so I use stitch markers now...every 10 stitches...and then I recount inbetween the markers BEFORE beginning to knit the first row.
Oy vey! I cast on 100+ stitches one time using stitch markers every 10th stitch...and knit away....and when it came time for some interesting pattern work...I discovered I had cast on either 1 too few, or 1 too many. Whatever it was, it ruined things. I frogged and started over.