If the area is very large use a long straight 2 in 4 in 5 1 cm 10 2 cm board and set your level on top of it.
How to level single floor board.
Raise the board until the level reads true then measure from the dip in the floor to the bottom of the board.
Cut a leveling strip a very long triangle from a 2 x 10.
Floors are rarely perfectly level.
A plywood subfloor will bridge any minor waves in the existing floor and leveling compound would help too.
You will have to make sure your joists can handle the addition of considerably more weight from the plywood subfloor and any floor coverings.
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.
In this video we show you everything you need to know how to level a floor into a perfect flat and level floor in your bathroom using self leveling floor com.
If the floor is uneven lift up the board until it is level so you ll know how much flooring compound you ll need to even it out.
Cut first leveling strip.