Watch David Vizard Powertec 10 Youtube videos. In one or two he references reversion. Combustion chamber face of the valve and not cutting a back cut on the exhaust are mentioned. David also mentions rounding the edge of the valve from the combustion chamber side to the margin to help the exhaust to not get caught on the square corner. This aids in keeping the valve cool. To prevent reversion it is probably a good idea to leave that little lip on the back side of the valve. It may help catching any reversion.