Probably;y telling you the obvious, before you chisel down the grain into the 'belly cut', tenon saw plenty of cuts at right angles and just above the 'belly cut' depth / profile. This video (just one I found on YT, nothing 'special') gives you the idea. https://www.youtube.com/watch?v=L2j57h3E3m4 This method prevents 'massive', ''unfortunate' splits along the grain etc, the kind of moments you do not want and makes it all easier. Apols if you know this and were going to do it anyway.