Who is associated with saying "back-of-envelope calculation?"

Dunno about the calculations part, but it has been said that Lincoln
wrote the Gettysburg Address on the back of an envelope.

He didn't.

from wikipedia:

The phrase is derived from the practice of quickly jotting down
calculations on the piece of paper nearest to hand, which in pre-email
days often meant the back of an envelope. Another possible origin for
the phrase is a widespread (albeit false) myth that Abraham Lincoln
wrote the Gettysburg address on the back of an envelope while on the
train to Gettysburg.

http://en.wikipedia.org/wiki/Back_of_the_envelope

Newspaper archives in the UK have variations of: back of the envelope
accounting, - arithmetic, - planning. Seems to start in the mid-1960s.

I agree with the above, just would add that there are similar
expressions about an  initial plan or calculation being made on the
table cloth or a napkin  - also the first available place to write
something.

I've just remebered another one: back of fag packet.