Given that the code is unlikely to be extracted soon, allow me to go on a whim and say that Z-move damage will continue to be calculated under the conventional damage formula, where *base* will be the base power as listed on Serebii. (If you check the Z-move pages on Serebii's Attackdex, it will list all the base powers for individual moves that have the potential to be Z-movified).

While I am willing to trust Serebii in their calculations, if anyone can prove that Z-moves use a different formula, I'd be willing to see your results.

Hope I helped. :)

Source: Above link and Z-move pages.