I don't think there is a fixed formula against the total vacation cost. If there were a fixed formula, It may be based on the flight itself. (Percent of full fare? Distance? Etc.)

FWIW, My MQDs posted have always been close to the actual cost of the lowest published fare at the time of booking. Could be a coincidence though.

So if the hotel/car portion is more expensive (I.e. longer stay), the MQD percentage seems to be a lower percentage of the overall vacation and vice versa.

My guess is Delta has an internal cost it applies to the air fare portion, and that's how many MQDs you get.