I know that various people use the proper tool from the likes of Race-Tech for getting fork oil height correct, but I found an easier and cheaper method.
When you've finished with the bottle of hand soap, keep the plunger bit.
Cut a length of plastic hose that you need for the air gap from the top of the fork leg ie. 120mm.
Attach this to the pump/plunger piece, lower into the fork leg and pump away...ensuring that you have some sort of receptacle to catch the un-needed oil.