I can't imagine how you'd multiply by 1.3 to make up for the extra taxi time. Unless you're doing some brutally long taxi's, or some very short sorties, I think the acceptable technique might be to add .3 to each sortie.

Think about it, I assume a 4 hour sortie in a Bone isn't unreasonable(?) If you were to multiply you'd be adding an additional 1.2 hours of taxi time. Which seems excessive.