hi freed i have 2.5 and 3m aluminium strait edges i pass them over the floor with a spirit level on top. but if ur floors that far out u should b useing slc
For a straight edge I use a 3m length of aluminium "I" beam used in demountable office partitions.
It's very cheap to buy one (or "borrow" it from a site) and they're very sturdy. Can easily be cut down to required length aswell.
Just put a level on top of it, spacers underneath ( 4" square ply offcuts) and sling it across the room to measure dips/high points.
ok so u find the high points and mark on the floor height? so what are the tolerances mounds above 3mm? 4mm? need be ground down or self leveller used?
If you have any humps bumps or dips in the floor save yourself the heartache and skim with slc its a better job in the long run.easier than trying to bed up with addy and risking hollow tiles.