I saw a Youtube video that posited that Holdo calculated to hit the ships at the moment before the ship entered hyperspace, when the ship was travelling at relativistic speeds, and applies physics to it to explain why it makes sense (and yet sort of doesn’t).
Nerdist: The Physics Behind The Last Jedi’s Coolest Scene! (Because Science w/ Kyle Hill)
https://www.youtube.com/watch?v=i1M95njhovwWith regard to how it doesn’t makes sense, he says that if the math works out, her ship hitting the fleet at relativistic speeds should have atomized the entire fleet rather than merely breaking the ships in half.
Hyperspace is another dimension. I think we can safely say that no living physicist can calculate what entry into another dimension would entail. Likely it wasn’t completely annihilated because the ship was only partly in this dimension so the power was reduced. Applying physics to science fiction and FTL is problematic.