I'm thinking of a number between 1 and 10

[Tavrobel]

e + pi

That's between 5 and 7.

I'm guessing "2 divided by 3: 6.66 - the number of the Devil", even though it's factually incorrect as the number would be 6.67.

That would depend both on how many significant figures you used, and which form of rounding you use; you can either use the standard "half and above" or you can use truncation. Also, 2/3 would still give you a value less than 1. So no, it's not factually incorrect, as there are multiple ways to reach a faulty conclusion that is based on interpretation.

On the other hand, we are making this way too complicated. No one has even bothered to guess "6."