How much zoom is needed for a distance of 600ft?
First you need to discover what focal length is “normal” for your camera. This will be a focal length about equal to the digital measure of the format frame. For a full frame 35mm camera commonly called an FX, the frame size (image sensor size) is 24mm height by 36mm length. The diagonal measure of this rectangle is about 45mm. If your camera sports a DX format, the imaging chip measures about 16mm height by 24mm length. The diagonal measure of this rectangle is about 30mm. Anyway, you can find this out for your camera by consulting the camera manual specification sheet.
Once the diagonal measure is known, you are on your way to answering your question for yourself. If you set your zoom lens focal length to about the diagonal measure, the view delivered is said to be “normal”. This translates to mean, the view delivered roughly equals the human view. In other words, what we experience with our naked eye.
OK, assume you have a modern DX digital; its “normal” is 30mm. Set the zoom to 60mm and the image delivered is magnified 2X times greater than normal. Now the subject at 600 feet images as if it were 300 feet from the camera. Set the zoom to 120mm and the object images as if it were 150 feet away.
I think you can now get the idea. Mount a 300mm on a FX “normal” 50mm and the subject will appear 300 ÷ 50 = 6 X closer. The object at 600 feet appears 6X larger as if it were 100 feet away. If working with a DX (sometimes called APS format), the “normal” is 30mm. Mount a 300mm and the magnification is 300 ÷ 30 = 10 written as 10X. With this lash-up the 600 feet away subject appears to be only 600 ÷ 10 = 60 feet away.