I try to get under 1.5 cents a mile, including all bonuses factored in (booking bonus, double miles). The upcoming $300RT (incl taxes) London trips I have work out to 1.25 cents a mile. The upcoming MSP runs work out to about the same. I wouldn't consider anything over 2 cents a mile just for building miles, but then I have a choice of 4 airports to depart from and probably have more options. If you're not getting double miles, maybe aim for something less than 3 cents a mile??
Tom in Las Vegas today