18" it says
your eqn is roughly same as Darcy-Weisman eqn:
hL = f(L/D)v^2/g
where f is the friction factor and L/D is the ratio of length to inside diameter.
It is not possible to calculate a friction factor from Bill's data using ONLY that eqn, because there is also a 90 degree turn so I assume you can set the eqn for calculating head loss in long bend 90 to the above eqn and then solve for friction factor. I find myself getting a bit confused by various subscripts associated with K at the moment.
For example, Crane lists K coefficients in tables as #*ft where ft is the fully turbulent friction factor but I would have expected K to = f(L/D) so if expressed as L/D then K/f would be equivalent not K*f). Gotta reread a bit...