Nitro beginner

I have mini kegs, three 64 oz and one 128 oz.I am looking at buying the nitro tap setup. My question is: Would I fill my keg with finished carbonated beer or flat beer?

Carbonated but to a lower level, say only 2 volumes of CO2.

To carbonate to a lower level, would I just use less priming sugar?
THANKS

Short answer… yes. Here is a priming calculator: