If the intake and exhaust valves are very different diameters, you may want to make two tools. The flat part of the cutting tool needs to be at least 0.050" longer from the centerline of the valve to the tip of the cutting tool so that when you make your cut, the valve relief will be at least 0.100" bigger in diameter than the valve head you will use in the motor.

Valves dance around in the valve guide bores more than you might think and you need to allow this clearance in the valve reliefs.