Reserve Fuel

[Covoxer]

If you have negative number of fuel used, than it definitely was added after you loaded your plane with FsP payload editor.
Maybe you were taxiing near by the refueling station in the departure airport?
Such things happend, so that MSFS refuels you to 100%.
Anyway, used fuel in FsP is calculated as fuel in load FsP dialog minus current fuel. If it's negative, that you have currently more than you
have loaded with FsP.
So...