Ahh, sorry then. Get yourself a cheap light meter then and keep your cannopy tight under that light. Try not to exceed a 1m x 1m footprint directly under it for your first grow. The light meter will show you how quickly the light falls off at the edges and will help you to set the height.
Don't get me wrong, all lights - even the old epistar blurple lights - can grow good bud. You just can't expect too much from them. It's like pulling a trailer with a small car. You have to size it correctly or you won't get up that hill.
