How is optimal Glide Path Altitude calculated?
While @JohnK’s answer is more accurate, the usual rule of thumb is 3nm/1000ft to determine when to start your descent. This “pilot math” is much easier and faster to do in your head, and the error is within the variation due to wind anyway.
In your case, the question is reversed, so 1000ft/3nm, or 333ft/1nm.