I find that I frequently want to adjust a recipe up or down in
quantity. I use an old slide rule to set up the original proportion (
set the 1 over the adjusted quantity; 2, 3, .5, whatever ) then read
the new quantity directly under the original quantity. Sometimes
there's a problem in measuring the new quantity exactly though.

Anyway I was wondering if anybody else does this or if I'm just an old
geek. I started using a sliderule studying electronics in the '70's.