B738 perf calculations and interpolating

[kadochen]

This might be a silly question and I kind of can anticipate the laughter and answers some of you will be posting. I'd nevertheless appreciate a hand:

You got a B738, a full load of pax and 50% of cargo capacity used on a flight that's taking 180 minutes, or 3 h. The bird is anticipated to use up 8.250 tons of fuel block to block. Now, let's say you get an additional cargo load which will result in 60% of the cargo capacity being used. my question is:

All other conditions, i.e. winds/temp aloft, flight levels, time to destination etc being equal,is there a linear correlation between the increase of the cargo load and the increase in fuel burn? In other words, does 10% + cargo result in 10 % + fuel burn? Maybe less? Is there, besides using standard aircraft performance calculation tools, a way to interpolate more or less accurately, or is it prominently dependant on aircraft types?

Thanks a lot for your thoughts.

[BOAC]

You will find a variety of 'opinions' on the figures, but somewhere around 3-3.5% per hour of the extra weight for 737NG. There are other factors to be considered but for ball-park stuff, try that.