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.
How to level a floor foundation.
In some cases pouring floor leveling compound will fix the problem.
Level floors are critical for reducing gaps between the flooring material and substrate and preventing sagging and other structural complications.
Nail them to the underside of the joists as near the foundation as you can.
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.
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.
Instead it may be an issue with joists and beams below your floor that require shoring up.
These footings support wood beams carrying the floor structure and walls above.
This video is for more experienced builders or for those who want an inside view of foundations and subfloors.
Your foundation may not be the problem.