Terumos 4:3-4

Terumos 4:3

A generous person should give one-fortieth of his crop as terumah; Beis Shammai says one-thirtieth. An average person should give one-fiftieth and a stingy person one-sixtieth. If a person took terumah and found that it was one-sixtieth of his crop, it is valid and he need not take more. If he did take more, tithes must be taken from what he added. If he took terumah and found it to be 1/61 of his crop (which is too small), what he took is terumah but he must take more to build it up to the proportion that he normally takes. Unlike taking terumah in the first place, such a correction may be done by measuring, weighing or counting. Rabbi Yehuda says that one may make up the difference even from produce that is not at hand.

Terumos 4:4

If one person appointed another his agent to go separate terumah for him, the agent must act as the owner intended. If he doesn’t know how much the owner separates when he takes terumah himself, then he should take one-fiftieth, which is the measure taken by the average person. If he accidentally took one-fortieth or one-sixtieth, the terumah is valid. If he intentionally took even one more than the owner would have, then what he separated is not valid terumah.