I hem all my pants so I pretty much eyeball my measurements. It doesn't have to be exact.
Put jeans on with footwear to gauge length adjustment. Fold one leg inward to desired length. Safety pin at inseam to hold fold. Left leg is folded outward to show how much needs chopping.
Turn inside out and check fold is even.
Line up legs.
Fold bottom leg to match top leg. Should look like this. Note that original seam is 2" from fold.
If original seam is less than 1.25" from fold then remove seam. The seams on this pant was 1-1/8". This is a pain so it's better to buy longer pants for easier chopping.
Optional: Iron to stay fold. Make sure to use either safety pins, pins w/out plastic heads, or temporarily remove pins as you iron.
Cut down to 1.25" or 1.5" (go wide for thicker jeans).
Trim off the little corner(s) of seam 1/2 way before ironed fold to reduce thickness for sewing. Be careful to leave inner seam alone.
Fold in 1/2 inward & pin.
Optional: Iron again to help stay fold.
Make sure you don't sew the leg closed. Using zipper foot helps keep even pressure while guiding the hem better. I use all purpose thread instead of buying thread for denim.
Newly hemmed jeans.