I worked out the same equation myself a while back, and it matches the equation you found. But I get one half the radius you get when I plug in the numbers: 16.5 meters. Did you use half of the running length for d, and divide by twice the sidecut depth?
I've used the same equation to estimate sidecut radius on some of my boards and it seems to give believable numbers. I remember getting a little under 9 meters for my old 156 Burton Alp and somewhere around 12 meters for a 167 Factory Prime which sounds about right.
....Wolfgang
