Cast on the 48.75” size instead of 42 to accommodate firmer gauge.

Need 100 sts at least around arm, 158-166 back sts

repeated rows 3 and 4- 17 times at 39rows--
95 back stitches, 42 arm stitches--
22 times left--
need 66 more back stitches, 60 more arm stitches--
every other purl row needs the increases!
Need 80 rows to arm separation.
Stop arm increases at 90 sts and cast on 12 extra

Stop body stockinette at 8” and add 1.5” rib (9.5” total)