How much 'fudge factor' does the a/c manufacturer build into the w/b graphs? Obviously if youre a pound overweight or the loaded moment is slightly out then the a/c isn't going to come crashing out of the sky.
Very true. There is a fudge factor built in to W&B, and it is described in the FAA flight test guide. When flight tests are done for certification, aircraft are flown slightly overweight, and slightly outside limits. But they are also being flown by a pilot who is extra skilled in flying safely through an unexpected event.
Saying that the plane flies "normally" in the configuration you're flying it might be overlooking a few details in the dusty corners, which might very suddenly become vitally important. Structural capacity, stall speed and performance are all affected by the gross weight. Add weight, surrender some of each of the three - how much are you willing to surrender?
Stall and spin recovery characteristics are affected by the C of G location. "Normally" in normal flying might be okay, but if the unexpected happens, the plane might not recover in the same why you've come to expect. I have flown a Cessna 206 in which at a certain, very attainable configuration, full elevator down control input would not lower the nose - that's scary bad, and C of G dependent. The aircraft was not misloaded - believe me, I triple checked!
When you pilot an aircraft, you're always thinking ahead, looking for the unsafe condition which you would rather carefully avoid than have to skillfully deal with. You're looking for the bad weather which could be ahead, the ATC clearance which might put you in conflict with another aircraft, the destination which might have too great a crosswind. Would it not seem odd, in that aura of caution and self preservation to blindly accept as accurate a W&B which seems too good to be true?
It is probable that the W&B records for the aircraft right back to the Cessna original are with the aircraft, or easily available. Sit on a rainy day, and go through them, doing all the additions and subtractions, and see if they pass the sanity check. As I have said, the empty W&B is the MOST screwed up things I encounter with aircraft - 'cause it is one of the more difficult things for the pilot to quickly recheck!