Can anyone give me the ground clearance of their car after fitting an air dam or replacement bumper to give me an idea of what I can get away with on track on a standard (preferably R3) suspension?

I ask as, out of interest rather than great expectations of looks or speed, I'm thinking of building a proper air dam and splitter for track day work with my R3 or my project car (a series 1 with 2/3" lower ride height). There's a lot of loose terminology so, to be clear, by 'splitter' I mean a horizontal plate going forward from a vertical air dam (the bumper extended downwards).

The splitter needs to have enough ground clearance for maximum braking with 18" wheels and track tyres such as Toyo R888s (grooved slicks have a lower height than the standard 225/45; more grip = more dive). I'm not too bothered by clearance on the road despite the speed humps we have around me.

I can cobble together a way of measuring what clearance I get in practice but if someone's got some info to save my time that would be a great help.