Incubator humidity quoted is essentially always RH - relative humidity - the amount of water that the air holds compared to saturation - which is 100%.
Any meter will always read RH, they may exist, but I have never seen or heard of a cheap absolute humidity meter - they are expensive laboratory instruments, not dime store items. Absolute humidity can also never be a % - a % of what? Absolute humidity is measured in absolute amounts - a weight of water per volume of air for instance, or dew-point, or water vapour pressure.
Electronic humidity meters, unless they are very expensive are very unreliable and very approximate. FAR more accurate to buy a good quality scale and use egg weight loss - a WAY better way to spend the money. Or use a wet-bulb glass thermometer - infallible, unlike electronic meters, and a fraction the price.
If you use salt solution to check a meter, the solution MUST be saturated - loads of crystals sitting in the bottom of the container, undissolved, and the container must be well sealed and the container and meter probe left to settle for as long as it takes to get steady reading - at least an hour I would say.
A correct reading at 75% though, is no guarantee at all that the meter will be correct at any other RH. A correct reading at some temperature other than incubation temperature is also no guarantee that the meter will read correctly in the incubator - the container with salt solution and probe need to be at incubation temperature to check things out - ideally in the incubator.
Unfortunately, absolute faith in cheap electronic wizardry is common, but very often misplaced when it comes to complex and critical things like incubation RH.
RH control is probably MORE important than temperature control - temperature control should be OK if the incubator is well designed, RH control is down to you. An egg has to loose around 12-15% of its weight during incubation - 2% either side and the chicks will struggle, and often fail, to hatch.
