Adding and Subtracting db of gain

Hey there, I am still wrapping my head around the way the units handle different suffixes etc.
I was wondering if there is a simple way of adding and subtracting say like 15 db to a signal prior and post another unit. My goal is to have a grid quantizers pre and post gain accessed inversely by a custom control in a custom unit in which the grid quantiser is embedded. However the pre and post faders on the grid quantiser itself are only so called “simple faders” ,that is, not modulateable and therefore not possible to map to the custom control.
I do understand that 15db is around 5.3 in terms of scaling on a vca but how do I divide the signal after the grid quantiser so I don’t get a level increase. (I could probably set the exact gain for both vcas one after the other but would rather be able to just turn a single knob increasing the gain on one of the vcas whilst decreasing on the other but with signal coming through all the time)
I am probably missing something obvious but any help would be really appreciated!