Well you want the actual trip formula here it is... but only when a light is perpendicular to the surface it is projecting on-
When the angle of the beam spread (degrees) = A,
The distance from the light to the surface (ft) = d,
And half of the beam spread in feet = x
The total beam spread in feet = 2x = s
since x is only half... the formula you are looking for is...
I suppose if you want I could explain why you have to do all that.... just not right now, I gotta get running!