Yes, there is math in custom knitting, and these armwarmers are no exception, but there's actually not a lot of math at all once you have your measurements and your gauge.
Multiply your number of stitches per inch by your elbow measurement to determine how many stitches to cast on. In my case, I got 67, which I rounded up to 68 to make an even number for ribbing.
Multiply your number of stitches per inch by your wrist measurement to see how many stitches you need to have decreased to by the end of the arm knitting. I got about 44. Subtract that number from your cast on number to determine how many stitches you need to decrease (24 in my case).
The first two inches of the armwarmer are ribbing, so no decreases will take place there. That means all your decreases need to fit in your arm length minus 2 inches; for me, that was 7 inches. Divide your number of stitches to decrease by your number of inches to determine how many stitches you need to decrease per inch. For me, that was a little more than 3, so I ended up decreasing 4 times each decrease round, which actually got me to 40 stitches at the wrist instead of 44 (a little extra snugness is fine; yarn can stretch).
If you know your rounds gauge you can plug that into your pattern, or just count rounds the first time and decrease evenly around the armwarmer however many times you need to every x rounds.
Once you've knit the length you need for your arm and decreased all you need, you'll need to increase again for the thumb. I did this by working an increase at the beginning and end of every other round 8 times for a total of 16 stitches for the thumb (my thumb is about 2.25 inches around, so that works with my gauge).
At the end of a round I slipped the last 8 stitches onto one needle and the first 8 stitches of the round onto another. Working just these stitches I knit 3 rounds plain, then 4 in ribbing and bound off.
Going back to the main part of the hand, I rejoined the yarn, knit 5 rounds plain, 4 in ribbing and bound off. These lengths will depend on the size of your hand and your preference for where the glove ends. I was going for right around the knuckles. The great thing about custom top-down knitting is that you can try it on and stop knitting whenever you're ready.