Here's what I do. I use a straight blade on my pocket knife. Ill try and scrape as much of the wall as I can, top to bottom. I try to keep it pretty much the same thickness. Once I start getting close to the sound I'm wanting, I will start thinning just the top half till I get the sound I want. As far as front to back I scrape the whole wall. Mine are a little thinner at the rear of the call than at the front. This allows me to get the a high pitch and rolls over to a raspy end not. That's what works for me anyway. Hope it helps. Waylon.