Noob question about XML variables

Greetings. I'd like to ask if anyone knows of a way to adjust the amount of fuel in a tank with an xml gauge?In the FSX sdk docs, it states that the variables for: "fuel tank center level" and its peers are "settable" variables, using a normalized percentage of maximum capacity as set in the aircraft.cfg file. It doesn't however state anywhere just how to "set" them.I'm able to use expressions to set normal "K:" sim events just fine, but this one is new to me. Is it possible to write to this variable through XML, or is it in the domain of simconnect only?Basically I'm trying to get an XML gauge to adjust the CG of an aircraft in realtime by adding or subtracting fuel or payload from various stations. Payload seems to be a no go, but this one? Boggles.Cheers, and thanks!