For a 45 degree helix the length of cut is the hypotenuse of the triangle or 1/cos(45) * linear length. The rotation is the wrapped length of the opposite side or (1/sin(45))*L*360/(pi*D).

Feed rate for the linear cut depends on the material and the form of the cutter. It seems to me that circular gear cutter cuts more like a saw than an endmill for initial calculations.

PathPilot is linuxcnc under the covers.