AerocatS2A answer is correct.
On a 3° GS you'll lose about 300' per NM. So when you are 15 NM out on GS you should be at 4500' above airport elevation (or rather threshold elevation).
Generally, to find the number of feet per NM descent, multiply the descent angle by 100. So with a 3° angle you will get 3x100 = 300'/NM. That leads us to 4500' at 15NM out on GS.
With a 2.5° angle: 2.5x100 = 250'/NM. At 15NM out on GS you should be at about 3750' above threshold elevation.