The Mishnah says: Masechet Avot, Perek Aleph (Chapter One), Mishnah Tet Vav. If somebody is asking you halacha (Jewish law) questions and you're unsure exactly what the answer is, you shouldn't just go with your gut or assume something; that would be going with the safek (doubt). Rather, you should have somebody to consult with, and not be ashamed to say, “I don't know the answer. Let me consult with somebody. Let me clarify the few points that I'm in doubt about.”
Rabban Gamliel haya omer, Rabban Gamliel used to say, Aseh lecha rav v'histalek min ha-safek. Make a rabbi for yourself or make a teacher for yourself. Go to somebody who knows the answer, and when that happens, you'll stay away from any doubt because at that point in time, when you clarify, you'll have certainty.
And secondly, ve'al tarbeh le'aser umadot. How much ma'aser (tithe) you're supposed to give, and give it to the right person. That's grown in Israel, and for other different types of people. So you get into a problem if you estimate. Even if you take too much in your estimation and you want to give the levi (Levite) more, instead of giving 10% to the levi, the crop that he has is still going to remain forbidden, because he didn't take the full amount that he was supposed to take from it. So the produce that he has is still not perfected. Then the extra amount that he separated away, vehistalek min ha-safek, or a teacher to ask questions to. For example, if you're a rabbi. That's one idea: if you make for yourself a rabbi, you'll remove yourself from any form of doubt, and remove yourself from any form of doubt.
Don't be in the habit of estimating, but measure exactly what the proper amount is. The Torah tells us that whenever we have produce, we have to separate terumot (offerings), ma'aserot (tithes), for the kohanim (priests) and for the leviim (Levites) before we're permitted to eat from the produce. For example, we have to give a levi 10%. You have to give 10% of the ma'aser to the levi. It has to actually be precise. You can't estimate less, and you can't estimate more. Why? If a person, let's say, separated too little, he only gave 9%. It's not metukan (properly tithed); it's still tevel (untithed produce), so his actual crop is still problematic. The ma'aser that he has in his hand, the amount that he took, is correct. It's 9%, but he's still missing a percent. That's if he estimated and took too little.
But if he separated too much, let's say he separated 11% of his crops, it is not going to have the status of ma'aser; it is not holding fully, because it's mixed between the actual ma'aser and the portion that's not considered to be ma'aser. So you have a mixture over there. Therefore, Rabban Gamliel recommends that a person should not estimate, but should actually count it out, because only 10% would have that status. He has to know how much ma'aser he's supposed to give, and give the portion that the person is holding exactly to the T. And therefore, you can't really treat it with the full sanctity of the ma'aser.