Exactly: it's the "perform your mission" part that tends to be defined differently by each different source. For example: the wikipedia article on the Spitfire Vb gives a combat radius of 410 nm and a ferry range of 991 nm. That means that this source is giving the Spit 171 nm of fuel to do whatever it needs to do (fight, loiter to join formation, etc). Other sources will increase the amount of fuel needed to do those things, and reduce the combat radius accordingly (a combat radius that's 40% of the ferry range means that only 20% of the range after landing reserves is being used for mission-related flying, a fairly low percentage).
20% climb/reserve is probably OK for low-level operations that don't need much formation building: recon missions definitely qualify here. If the operation is taking place at higher altitudes or are large, a larger climb/reserve is probably a good idea.
This post has been edited 1 times, last edit by "Hrolf Hakonson" (Dec 31st 2008, 4:37pm)