you can always check with the fuel and payload tab from fsx itself...cog will be the percentage of whatever the crosshair occupies counted from the nose of the aircraft and a total of 32 i think..so if the crosshair is at the 16th line from the top then its 16*100/32 which is 50%..and so on....
i am not sure on the amount, i just think it's 32 from the datum, you may want to confirm that forst
hope that helps.