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I think this assumes the order of magnitude of the size of the observable universe.

Clearly if the diameter of the universe is smaller than a proton then you don't need 43 digits of pi to calculate its circumference to smaller than the diameter of a proton. So if the diameter of the universe is 10^1000000^100000000 proton wide the precision you need for pi would be way higher than 43?

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It does depend on the size of the observable universe, which we do know because, well, it's observable.

The post i replied to started off with "If you know the diameter of the observable Universe" but if the result it depends on the size of the observable universe then you ought to say "given the size of the observable universe".

Starting with 'if' would imply that the result only requires you 'knowing' the size of the obsv. unv. but does not depend on its actual size.

I think the meaning of "If you know," in this case was more like, "If you take as an input."

Yep. "If you know X" is commonly used as "given X".

There's a difference between "the size of the universe" and "the size of the observable universe", see also "cosmic microwave background", "dark energy", and "inflation."

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