[hanniabu]

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

[diskcat]

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.

[singlow]

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

[coldtea]

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