It is basic geometry, as has been mentioned.
But what is the operational use in knowing this figure or spending time to calculate it? Will knowing this number make any difference to You on the approach, seeing that You will get a RVR/visibility and a ceiling or VV provided? Maybe I have missed something until now, but I never felt the need for such a number.
If You want to determine the visibility on the ground on the runway, why not count the runway centerline lights? The number seen times 15 will be the vis. in meters; if other units are preferred, just apply a factor.