Floor clearance for downfiring driver ?

I've seen this topic raised before but have since seen some manufacturers that throw the general rules out the window.

I've always had a height of about 100mm [4 inches] off the floor in my mind for no other reason than a hunch.

Some REL subs have a mere 50mm and some of the feet you can buy for subs range from 50mm to 150mm or if you want to add furniture legs, as high as you want.

Is there any golden rule apart from it being high enough to stop the driver not bashing into the floor!
If the subwoofer is vented and the port is also downfiring, some use the clearance to physically extend the port, to achieve a lower tuning point.

If it's sealed, I don't know of any other rules, except obviously "being high enough to stop the driver not bashing into the floor!"

OTOH, I think if it's too close you may run in some problems, air loading will be stronger in front of the cone than in the back.
One possible criterion is that the area of the cylinder around the periphery of the driver should be comparable to the driver area - this ensures that air velocity between floor and box is no higher than that in front of the driver. This may be stricter than needed, but is a starting point.

Now if r is the driver radius, the driver area is pi * r *r. If h is the height of the driver above the floor, the peripheral cylinder has area 2 * pi * r * h. Solving for h at equality, this says that h = r/2. For a 10" cone size driver ( this would typically be called a 12" device), you get a spacing of 2.5 inch; a 15" nominal (maybe 13" cone size) wants 3 and a bit inches.

No idea if my criterion is the right one, but I expect the scaling is valid - big drivers want a bigger gap.
I agree with Piglets.
gap ~=Diam/4
Tuning around this gap value may produce some change in the way the driver feeds the room.

I guess a smaller gap equals more resistance = lower Q and may give a less boomy response.

A bigger gap allows the driver to perform nearer the model predictions used in designing the box.

The vent, if fitted, needs a gap of at least 1.5*Diameter or the vent tuning will be altered.