@Mad_X Check out item 3 of a write up I've made a few months back.
One Request, One Observation
It's almost identical I believe as yours. But the funny thing is, my fuel used did agree with the tables. Just not the indication. Have a look at item 3 of my post and try to do the same. Doesn't need to be as complex. You can take remaining fuel at the start, then (leaving power constant) take another reading 15 minutes later. That difference in fuel remaining, multiply it by 4 to get your 2-engine fuel burn in pounds/hour. Divide that by 2, and you get the average engine fuel flow. That value should match the fuel flow indicator (and also your tables).
For me, it matched the tables, but not the indicator. As you can see from my post, I didn't hear anything back other than that it should work normally.