Specifies the nozzle's exit diameter.

Specifies the ratio of the throat area to the exit area. This number must be in the range 1.1 to 8.0.

You may input either the nozzle exit diameter or the area expansion ratio, but not both. The web page will compute the other value.

Specifies the angle at which the sides of the nozzle diverge at the nozzle exit. If the nozzle is straight-sided, as are most amatuer nozzles, the half-angle is the angle between a line running straight through the throat and the edge of the nozzle cone.

Values greater than zero reduce the motor's effective thrust because some of the exhaust gasses generate off-axis thrust which does not contribute to overall thrust.

Most amatuer nozzles use a 15 degree half-angle.

This parameter adjusts the nozzle's characteristic efficiency. A value of 1 describes a perfect nozzle. Smaller values describe less perfect nozzles.

Lacking an instrumented test stand I do not know what values to use for this. I believe that a value in the range 0.85 to 0.95 is reasonable.