Focal length refers to the distance from a point within a lens to the film plane when the lens is focused on infinity. For a 50mm lens, this point (rear nodal point) is 50mm away from the film plane if the lens is focused on infinity.
An image on the ground glass of a view camera is focused by racking the lens board towards or away from the glass, but the principle is the same. The rear nodal point of a 250mm lens will be that distance from the glass when focused on very distant objects.
It is virtually impossible to physically measure the focal length if that's what you're trying to do, since the rear nodal point is contained within the rear optic. If you're trying to calculate it, that's a subject beyond me, and perhaps the photography category also. In that case, you might want to try asking somewhere over in the Physics category.