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.
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.
You will have to make sure your joists can handle the addition of considerably more weight from the plywood subfloor and any floor coverings.
Raise the board until the level reads true then measure from the dip in the floor to the bottom of the board.
On the top side of the floor another fix it idea to lay down new hardwood over the existing floor.
Cut a leveling strip a very long triangle from a 2 x 10.
Floors are rarely perfectly level.
Cut first leveling strip.
A plywood subfloor will bridge any minor waves in the existing floor and leveling compound would help too.
Rather than attempt to lift the floor by packing out between the boards and the joist the packing invariably works loose as the floor moves with use it is easier and more cost effective in the long run to remove the floor boards and level the joists by fixing new straight ones to the existing.