With
all due respect, you don't need a scale. Just a calculator.
Take
the main number of the scale of the plans you have. (For
example, if your plans are 1/72, you could actually divide 1
by 72, but it's easier just to use 72. It won't affect the results.)
Divide
by the scale of the plans you desire. (e.g., for 1/144, use 144.)
Multiply
by 100 to get a whole-number percentage increase or decrease.
72/144=.5
; .5 x 100 = 50(%)
Run
your 1/72 plans at 50% and they're 1/144.
It
doesn't matter which number is larger or smaller, as long as you geth
them in the right order. If you've got 1/72 plans you'd like to make
1/32, 72 divided by 32 = 2.25 x 100 = 225%
It's
not even math. (I hate math, like most everybody else.) It's just arithmetic.
Cheers