First you need a solid concrete footing.

Got a good floor under there - concrete

Dont use hydraulic jacks if
your jacking will effect diferent floors

What do you mean? If I don't have a concrete one?
And why are screw jacks better?

Is it just a
sagging floor or an unlevel area , how big.

Tomato, TomAHto. Aren't they the same thing?
Most of my main 2 rooms (small house - each room
is 15' x 15' roughly) sags towards a point near the
center of the two rooms