Yep, Grog's got it. Those fixtures are adjustable. You can flood them all the way in or out. There is usually what is known as a Mulitplying factor that is available for fixtures that lets you figure this out, but in a situation like this, you can't really do it. Same thing with a
zoom. It's adjustable between a certain degree measurement and you kinda have to guess and play with it some. You know what the max and the min are, as long as the size of field you want is between those, then you are ok.