You can also calculate the distance the water travels by applied physics, if you assume that the water jet behaves as a projectile. Since d=at^2/2, where a is acceleration, then if we replace a = g, and d = H, then, H=gt^2/2, and t=(2H/g)^(1/2), and the horizontal distance X the jet travels is X=Vt.
I usually calculate the distance for the top of the jet, and provide a margin of error in the rip rap length to allow for possible errors in the velocity calculation.