I was curious if there was a way to update/alter upkeep, as it doesn't seem to match that calculated in-game.

I had created a starter (stone 1x2 with triangle, 2 double doors (sheet), one double doorway unfilled).
Rust gives upkeep at 15 blocks, 10% upkeep
Fortify gives upkeep at 17 blocks, 10.6% upkeep

Looking at this, it would appear that Rust does not include doors (and windows) in upkeep calculations for number of blocks, but does for cost, while Fortify includes them for both block count and cost.

Is there a manual way to alter this with Fortify, is this something which may be patched in the future, or is it a quirk on my server?