Ech… ok, let me have a try (sorry for the crap rectangular illustration!):
Lets label the rectangle as “ABCD”,length a=AB=CD, and
width b=AD=BC,
where a>=b
centre of the rectangle is lablled as “O”
Folding A up towards C means a reflection A<->C on the crease PQ
So, the crease PQ is perpendicular (at an angle) to AC at O.
From the similarity of triangles APO and ABC, we can say that:
PO : BC = AO : AB, or PQ : AC = BC : AB.
So:
PQ = (b/a) x squareroot(a^2 + b^2) = b x sqareroot(1 + (b/a)^2)
Yeah, that’s right… i think…or i just confused myself 