If anyone could please help me out on this....it's a bit too tricky for me...but i may be overlooking a simple shortcut.

Here's the deal, we have to keep track of the levels of fuel in our trucks. the main truck holds 2200 gallons of fuel when "topped off/full". throughout the day, we pump fuel out of the truck, causing the level to drop.

Example:

<<2200>>
-100
-50
-400
-50
(1600) --"running total"

what we are having trouble with is when we put <<2200>> into the picture, we want it to say 2200 gallons for the running total again. preferably, we would like to say "topped" and have it automatically change the running total to 2200 again.

can anyone suggest anything?