You need two to three times the height of the bulb of soil above the top of the bulb. Crocus corms are usually about half an inch deep, so plant them a good 2 inches in the soil so that there is about 1.5 inches of soil above them. Then on top of that you will have your gravel mulch. Crocus corms will move themselves to the depth they like to be at by the way.

You could plant them in pots & then plunge the pots to the correct depth, then when they have flowered just pull the pots up & put them somewhere else to get on with all the leaves that they produce. Keeps the gravel looking tidy.

Thanks folks, much appreciated!