Lemurian & kijangnim,
I'd never heard of the K factor so I stand educated. *
We usually left tankering decisions to you management chaps anyway
As far as extra burn is concerned we kept it simple:
3% x hour for extra loaded.
4% x hour for extra planned arrival.
and thanked our lucky stars we didn't work for Ryanair.
( * Edited to say except for K in calculating SAT from TAT)