The main one I want this for is the product output field

I am thinking nothing more the basic +-*/

example use case
the current product amount uses 2.687 machines, if I do
(current output) /2.687 then
(new output) * 3
I will have exactly 3 machines required

example 2
the current product amount required 20 items per sec in, if i do
(current output) / 1.333
then I will input exactly 1 belt.

This is a good idea, and has indeed been on my todo list for quite a while. It's not super simple to implement though, so I'm not sure when I'll be able to get to it.

i think one of the best ways to implement it would be with the lua string library, in particular

lua patterns
https://www.lua.org/manual/5.4/manual.html#6.4.1

and the for loop shown in string.gmatch
https://www.lua.org/manual/5.4/manual.html#pdf-string.gmatch

it is a good mod, but there are a couple pieces of friction like this where it still lacks polish.

Sure :) It's not just the math that's the issue, it's also the GUI that now needs to be a bit more custom in some ways.