Terumos 4:5-6

Terumos 4:5

If a person wants to give more terumah, Rabbi Eliezer says one-tenth is the maximum, as is the case with terumas maaser. If he separated more than this, he should designate it as terumas maaser for produce that he has elsewhere. Rabbi Yishmael says he may make his crop half terumah and half chulin (non-sanctified produce). Rabbi Tarfon and Rabbi Akiva say he can give as much terumah as he likes so long as he retains a portion of it as chulin.

Terumos 4:6

A person measures his baskets three times: when the first produce ripens, when the late summer produce ripens, and mid-summer. One who counts his produce (to know how much terumas maaser to take) is praiseworthy; one who measures it is more praiseworthy, and one who weighs it is the highest-praised of all three.