SpongeBob SquarePants sitting atop my air cleaner lid

Simple physics, when something wet and a little sticky touches something else, the material transfers from one object to the next. Put some GOJO soap in the palm of your hand, and take the index finger of your other hand and just barely touch the soap. Your index finger now has a little dab of soap on it.

Keep decreasing the height of the sponges until there is no transfer of the soap when you close the hood, measure the thickness of the sponge stack at that interval in the test, and that is your clearance.