I use a 2” masonry holesaw to get to the substrate, if your lucky it’s PlYwood. This gives you a flat reference. Mark your four bolt locations as you normally would. Use the circle cutter and get to the substrate. Measure from the substrate to the stone that protrudes the furthest. This gives you how far you need to space out. Personally I use fender washers to build out the distance needed. To make it easier I stick them on a bolt and layer them until I get the measurement and wrap it in electrical tape.

This ensures your mount is perfectly flat in reference to the substraight and your not relying on the stones/lickandstick holding the panel. It really doesn’t take that much longer doing it this way then dicking around with useless tapcons and trying to pad it out with washers.