rule of thumb

rule of thumb (a method or procedure derived entirely from practice or experience, without any basis in scientific knowledge; a roughly practical method; also, a particular stated rule that is based on practice or experience; a useful principle or a general rule which is good to follow; {of persons}: working only by methods derived from practice) — повседневное правило (основанное на опыте, а не на научных знаниях); практический способ; житейские соображения; ||≈ приближённый подсчет; кустарный, доморощенный способ; на глазок (Hence rule-of-'thumbite (a person who works by rule of thumb) — работающий кустарным способом, на глазок)

by rule of thumb — эмпирически; приближенно; на глазок; опытным путём; ~ по старинке

as a rule of thumb ≅ практика показывает, что (нужно, полезно, лучше, др.)...

Example 1: When cooking fish, a good rule of thumb is 10 minutes in the oven for each inch of thickness. (eslmania.com)

Example 2: As a rule of thumb, plant carrot seeds two inches deep. (native-english.com)

Example 2: Ted, as a rule of thumb, you should always plan to study for your chemistry tests for at least two hours.(eslmania.com)

Example 3: I never weigh anything when I'm cooking — I just do it by rule of thumb. (ALD) — Я никогда не отмеряю продукты, когда готовлю. Обычно кладу все по вкусу.

Example 4: The general rule of thumb is that one portion size is about the size of your fist. Use this as your guide to determine how much you should eat at your local restaurant. (How to Avoid Overeating when Dining Out)

A rule of thumb is not exactly a rule, it is more like a good idea.

Example:I don't invite people to my home unless I know them well, as a rule of thumb.

You don't have to follow a rule of thumb because it is just a good suggestion.

Example: "Do you always have to use white paint?" Reply: "No, that's just a rule of thumb. You can use whatever color you like."

If you have a better idea then you can do it your own way but it usually safe to follow a rule of thumb because it has worked well for other people. (goenglish.com)

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Definition good idea to use;

Definition
good idea to use; rough guideline

Examples from englishbaby.com

..... As a rule of thumb, I always bring an umbrella when it’s cloudy. You never know, it might rain.
.....It’s a good rule of thumb never to trust anyone with your passwords.
.....It’s good to arrive late to parties and early to job interviews. That’s my rule of thumb.