There is no need to recalculate, because the mass of the big black hole is already included when you measure the mass of a galaxy. A black hole is the same gravitational field as stars of the same mass, but its gravity is concentrated in a very small region by comparison.
The mass distributions in galaxies are partly found using rotation curves, which tell you how fast objects are spinning around the center. These curves tell you how the mass is distributed, and whether the center is a bunch of stars very close together, or a big black hole of the same total mass, the answer is pretty much the same. But because many galaxies looked like they had a very massive center which was not plausibly star material, it has long been suspected that most galaxies harbor supermassive black holes.
The discovery that the Milky Way's black hole is very important astronomically, because we can't directly see it, and it is in a very difficult to observe the direction toward the center of the galaxy, because it is obscured by galactic dust. But it is a confirmation of results long expected. It would have been more surprising for astronomers to not find a black hole at the center of the galaxy.
The difference is the black hole has no luminosity, but the dark matter is apparent in the rotation curves, it comes from a diffuse cloud of stuff around the galaxy. Further, the same diffuse cloud of dark stuff clustering under gravity is necessary to explain how galaxy clusters could be stable, and the formation of gigantic filaments like the great wall of galaxies.