OVERWT TKOF.

Narrative

EARLY MORNING DEP ON THE FINAL LEG OF A CONTINUOUS DUTY OVERNIGHT; 5 HRS PLUS 30 MINS AWAY FROM AIRPLANE ON OVERNIGHT. AUTO FEATHER WOULD NOT PASS AUTO FEATHER TEST. CONSULTED MEL AND PERFORMANCE CHARTS TO DECIDE WHETHER LEGAL TO DEFER AUTO FEATHER. CHARTS INDICATE 7080 LB WT PENALTY FOR RWY LIMIT AND 12980 LB WT PENALTY FOR CLB LIMIT. I MISTAKENLY USED THE RWY PENALTY WHEN I SHOULD HAVE USED CLB LIMIT PENALTY. I HAD BEEN TAUGHT THAT THE CLB LIMIT RESTR WAS ALWAYS THE CORRECT PENALTY TO USE. I WAS CONFUSED BY THE RWY PENALTY EVEN BEING PUBLISHED. IT CAN NEVER BE USED SO WHY PUBLISH IT? ALSO; AT A XB40 SHOW TIME FOR AN XC10 DEP FOLLOWING A XA53 ARR THE NIGHT BEFORE. WE WERE IN THE HOTEL FOR A BOUT 5 HRS. AFTER DEP; I NOTIFIED THE COMPANY THAT I HAD DEFERRED THE AUTO FEATHER AS PER THE STANDARD PRACTICE. UPON ARR AT THE DEST THE CHIEF PLT ASKED ME TO SHOW HIM HOW I CALCULATED THE TKOF PENALTY. I SHOWED HIM AND HE THOUGHT IT WAS CORRECT. HE CALLED DISPATCH AND THEY INFORMED US BOTH THAT THE CLB LIMIT PENALTY WAS THE CORRECT PENALTY; NOT THE RWY PENALTY. I FILLED OUT COMPANY UNUSUAL OCCURRENCE RPT. MY ERROR CAUSED APPROX A 4000 LB OVERWT TKOF FOR CONDITIONS.