I was having troubles to see how thick the slab was going to be on the live edge side. It is not posisble to mark this with a marking gauge, because the pin is just too short. So first I deviced this method, sawing curves down to the proper depth. I meassured the depth with a combination square, marking a tick with a chisel. Then I would cut down into the edge with a saw.
While helpfull, this was not precise enough, especially not in between the lines. And it was difficult to see while planing,how deep I was going to be. So, I decided to make a special marking gauge. A block of wood, cut out at the desired depth with an old planer blade on top for a marking knife.
I could even move the blade for two different reach settings. Shorter reach is more precise of course, because the blade tends to curve under pressure. But despite this shortcomming it works pretty well.