Find a good Attitude / Thrust setting table of the airplane you fly.
Usually, it contains Attitude / N1% / IAS / Configuration and FUEL FLOW data.
Just take a look at the Fuel Flow with Flaps XX. A good example in the following great document (B737 Classic):
http://www.b737mrg.net/downloads/b73...ersettings.pdf
Flaps 5 Level Flight is 1200 Kg/hr while the Clean one is 1000. So It is 20% penalty.
Gear Down Flaps 15 is 1500 Kg, so 50%. I can assume Flaps 10 could be 30-35% due to the LED going to Full Extend.
It is a good quick hint in a busy situation. If you have time just cross check it with the Actual Fuel Flow or....at the next flight make some tests
