When you calculate and determine (the calculation), you are determining the calculation. That's why this word is to calculate.

Unlike 計算する, which means “to calculate” in general, 算定する is a formal word that focuses more on the final value of a calculation, instead of the process of calculating itself.

This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.