If Mega Mawile could still reach the same attack stat of 678 without Huge Power, how high would its base attack have to be?

EDIT: Include the formula used to calculate it.

Base stat is defined by the following formula for Attack, Defense, Speed, S Defense, S Attack, for a boosting Nature:

[(678/1.1) - (252/4) - 31 - 5]/2 [616 - 63 - 31 - 5]/2 517 / 2 = Base 259 Attack