When you use your intuition to determine the amount of something, you'll end up doing the calculations in your head. You do the same thing when it's time to pay your bill at dinner too, just determine the amount in your head, go with your intuition, and it should be the right amount.

This is a jukugo word that uses the on'yomi readings of the kanji. You didn't learn this reading for 定 though, so here's a mnemonic to help you:

If you're having trouble with the calculation of the amount on your bill, just call Joseph (じょう) Stalin. He's really great at numbers and he will be able to figure out the numbers in his head really fast.