I did a quick search and saw that there were many threads regarding increasing fleet supply but I'm not interested in that. Instead I would like to know if it is possible to alter the way in which fleet supply and upkeep work.

My thinking is that your income loss from upkeep should be proportional to the amount of fleet supply in use.

For example, let's say you've researched the fifth level of fleet supply, 820/38%, but are only using 580 fleet supply. As it stands now, you are in effect paying upkeep on ships and a fleet that don't exist. Your upkeep costs are the same regardless of whether you are using 1 or 270 points of that level of fleet supply. 

The math:

[(remaining fleet supply) - (fleet supply level)] / (fleet supply level) = (FS usage %)

(upkeep level %) * (FS usage %) = (applied upkeep %)

So:

[(240) - (820)] / (820) = (70.7%)

(38%) * (70.7%) = (26.9%)

Using only 580 of your 820 fleet supply would get you an upkeep income loss of 26.9% rather than a flat rate of 38%. Similarly, using 1680 of 2000 fleet supply would result in an upkeep of 63% and using 360 of 2000 would generate an upkeep of 13.5%.

Is this possible as a mod or is it a feature that's hard coded into the game? If it is possible, where would I start?