It does not do to leave a dragon out of calculations...

I don't know the context, but the sentence seems to imply that when you know of some potentially dangerous situation that can mess up your plan A, you'd better have a plan B.

So what is the exact meaning of "leave...out of calculations"?

Keroro
So what is the exact meaning of "leave...out of calculations"?

If you "leave something out of your calculations" it means you do not consider it when making your plans.

KeroroSo what is the exact meaning of "leave...out of calculations"?

fail to consider; neglect to take into accout

CJ

KeroroSo what is the exact meaning of "leave...out of calculations"?

fail to plan ahead for X.

" It does not do to leave a dragon out of your calculations, if you live near him."


What does "it doesn't do" mean here? And is it common?

Thanks.

"That simply will not do!" is a little bit old fashioned.

"That is not among the possibilities/options!"

"It doesn't do to X." = "X should not be done." "It's not acceptable to X."
"It does not meet with propriety" (with practicality," in the case of the dragon.)

Of course it's an idiom.

I think of an old aunt/grandmother knitting in

Thanks a lot Avangi. I should have understood that it's an old saying because Tolkien wrote it

The verse is about risk planning. In planning to escape a dragon, acknowledge the risk of getting caught by a dragon.

Understatement, a way of saying, "The dragon is the danger to plan around."