I did my adjustments by unpicking the thumb and re-knitting it so here is the adjusted pattern which should knit up really nice. When casting off at ends and casting off I always like to keep the stitches loose this gives for a better fit and more stretch when wearing/putting on/taking off.
Tension square 1inch = 5st x 7rws
Co 39st split 3dpn 13-13-13
Rw1-7: k1, p1
Rws 8-23: knit
Rw24: Inc1, knit 9, RPT to end of rw. 15-15-14
Rw36: inc 1, knit 7, move next 9 to holder, move needle 2 to holder, move needle 3 first 7 stitches to holder and knit remaining 7 stitches rearranging needles 5-4-5.
Start new row count for thumb
Rw1: knit 5-4-5
Rw2-7: k1, p1
Rw8: cast off.
Rejoin picking up two stitiches from base of thumb, redistribute needles to start 4 across from picked up st. (From 6-15-13 to 11-11-12)
Rw 12-17: k1, p1
Rw18: cast off.