Kinda hard to explain, but on a paper, three scales are printed, looking sort of like slide-rule scales. Numbers. Say the top one is gross pay, the middle one is a percentage, and the bottom one is net pay. You lay a ruler across any two, and read the answer on the third.
There’s a specific name for this kind of computational thingie…and I don’t know what it is.
Nomograph or nomogram.
Got it in one. AKA the nomogram. See Nomogram - Wikipedia
Nomography pages associated with the Atomic Rockets website:
Relatedly, there are lookup tables with a slidable cardboard window to only show one row at a time. A lot of teachers used to have something called an EZ-Grader, for instance, which you could set for a total number of questions, and see at a glance which number corresponds to what percentage.
There’s a novel (or ongoing serial, I’m not sure which) by one Rob Garitta called “Numbers Running”, excerpts of which can be read on the Twilight Of The GM blog site. It concerns a backward (by interstellar standards; their indigenous production base is roughly equivalent to early-mid Twentieth century Earth) planet called Zaonia. When Zaonia is embargoed by the equivalent of the Trade Federation, the people there rediscover pre-electronic ways of doing things, including log tables, slide rules, analog mechanical computers and nomograms; with which they can do the math necessary to get independent tramp freighters into orbit and back again.
They are called income tax scales.
Hmmm. Where does a “vernier” fit into these definitions? Or are my brains too scattered?
~VOW
A vernier is used for very precisely measuring the position of one object along another, such as the two pieces of a micrometer. It consists of a pair of scales with slightly different spacings, such that the two will line up only at a single tick mark.
reminds me of college days (many many years ago) when everybody reluctantly converted from Cycles per Second to Hertz. Someone created a handy guide to conversion, including a semi-log graph and a nomogram. 
Can we assume this is a whoosh? Lemme check my calculator, but I always converted from CPS to Hz by removing the “CPS” and substituting “Hz”. Too esoteric?
Close. But you left out a crucial step.
You need to multiply CPS by 1.00000 to convert to Hz and conversely you need to divide Hz by 1.00000 to convert to CPS. 
In the pre-calculator era a slide rule or nomogram could be very helpful for this stage of the conversion.
/Was/ a woosh. “reminds me of college days”
Little-known secret: you can use the CPS-Hz nomogram to implement ROT-26 encryption. It’s twice as secure as the ROT-13 cypher.
I’ve encrypted a message using ROT-26 below. Decryption via nomogram or supercomputer is left as an exercise for the reader. Here’s the encrypted message:
Be sure to drink your ovaltine.
Surely the rot-26 encryption of that would be
“bE SURE TO DRINK YOUR OVALTINE.”
“E-bay ure-say o-tay ink-dray our-yay valtine-oyay”?
Not to nit-pick, but that only works for metric CPS.
For English unit CPS, of course, it is not possible to convert to Hertz.
I’m sure you already knew that, but I didn’t want any casual readers getting confused.
To clarify, the conversion is reciprocal for Imperial CPS:
You need to divide Imperial CPS by 1.00000 to convert to Hz and conversely you need to multiply Hz by 1.00000 to convert to Imperial CPS.