Charts for Ferrite Inductor Design
Courtesy of Fair-Rite Magnetics
Kfactor = lr/lc^(lc/lr^(0.42517))
The red line indicates a value of 1.77 for a ratio of 0.45 (45% of the form length). Therefore, the coil is positioned slightly away from the left end . A 3/8" diameter rod would be about 5 mm away.
