If you suspect that you re working with an off level foundation you can get it ready for installation quickly by using a floor leveling compound.
How to level a floor foundation.
Your foundation may not be the problem.
If an engineer found no structural issues you may be able to level the floor by applying a cement based self leveling floor product such as henry level.
For instance you may have a dining room floor that end to end is level but between those two points are various sags and dips.
A pier and beam house foundation essentially elevates the house above the ground to provide a level platform based on specifically positioned footings.
These footings support wood beams carrying the floor structure and walls above.
Level floors are critical for reducing gaps between the flooring material and substrate and preventing sagging and other structural complications.
This video goes over some basic foundation and.
You may find this task easier if you hold the string off the sagging floor joists by attaching blocks of the same thickness at opposite sides of the basement.
Nail them to the underside of the joists as near the foundation as you can.
Instead it may be an issue with joists and beams below your floor that require shoring up.