Let look at it from two sides, the first is the aircrafts actual performance, if you have a 10 kt tailwind the calculation is actually based upon 15 kts by regulation. So the weights/V-speeds in the 10kt column are actually for 15kts, this is a conservative method of protecting you against a higher tailwind. It’s therefore fair to say that if the wind is less than 10 kts, you are still perfectly safe.
The second side is the “what if” scenario, for some obscure reason you find yourself sitting in front of an accident investigation board, they have had the time to review your FDR, your takeoff calculation and the weather, they also can review your companies and manufacturers operational procedures. How are you going to justify using the tailwind data for zero wind, what written procedure did you follow? Would you only apply your personal procedure to zero wind conditions or would you apply it with a 5kt, 10kt, 20kt HEADWIND? Its an extremely gray area and I believe that you will find that both your company and the manufacturer will hang you out to dry…….
So I suggest that as there is NO procedure that allows you to use the 10 kts tailwind data for zero wind conditions, you don’t do it!
Mutt