How to translate "three inches"?

When I was watching STRANGER THINGS on NETFLIX, I found there was something kind of wrong with the translation.

I watched the show in English with Japanese subtitles.

There was a scene where Police Chief Jim Hopper was terribly anxious about what his adopted daughter El and her boyfriend Mike were doing in her room. He told El, “Keep the door open 3 inches”, while the Japanese subtitle said, “Keep the door open 7 centimeters.”

What made me feel uneasy was “7 centimeters”.

An inch is about 2.5 centimeters. So, 3 inches is 7.5 centimeters, but the translator rounded it down to 7 centimeters.

I guess the translator must have felt that mentioning the decimal position is unnatural in everyday conversation. But, if you ask me, “7 centimeters” is still unnatural in Japanese. I would have translated it as “10 centimeters”.

If you put 7 centimeters and 10 centimeters side by side, the difference might seem huge. True, Jim said 3 inches, but that number did not mean anything significant. It just meant “a little”.

When speaking in Japanese, if someone says, “Keep the door open 7 centimeters”, he will probably be asked, “Why 7 centimeters? Why not 10 centimeters?”

If Japanese people mean “just a little”, they say “honno 5 senchi ka 10 senchi (just 5 centimeters or 10 centimeters)” or “shichi-hassenchi (7 or 8 centimeters)”.

That is, 7 is not used alone in Japanese if it is not precisely 7. It is often used together with 8.

Well, let's look at another example.

In the same show, I heard the kids talking about the distance to somewhere. One of them said, “It’s 3 miles ahead”, and the Japanese subtitles were “Almost 5 kilometers ahead”.

A mile is about 1.6 kilometers. So, 3 miles make 4.8 kilometers.

This translation sounded right to me, while I was a little uncomfortable with “7 centimeters”.

I think it is because “3 miles” is the actual distance, while “3 inches” is just an analogy, and when used as the analogy, 5 and 10 are nice crisp numbers in Japanese, but 7 is not.

Anyway, translating is a very difficult job, I think. It is not just merely the conversion of numbers.