Tom and Brian, several bell mouths are shown on the attached. The elliptical profile reflects a soft wave and it is best if wave dynamics cannot be calculated or utilized, or if a wide power band with no humps and dips in the torque curve is the the goal. The simple radius reflects a sharper wave and it is best if wave tuning can be done and be taken advantage of. It will result in spikes and dips in the torque curve. This can be a big asset if the spikes can be utilized.
The simple radius bell will be used on this build with 1/4 inch radius. The tuned length will be based on the fully contracted cross-section 1/4 inch inside the bell mouth. The average distance between this section and the valve seat will be the tuned length. This is the average of the distances along the bottom of the port, the distances along each side, and the distance along the top. This is what the users manual recommends as best as I can interpret.