1. If you divide difference in altitude by rate of descent, you get time in minutes. You then multiply this by ground speed, dividing by 60 (since ground speed is in knots = NM/h).
Track distance = (Altitude difference / Rate of Descent) x (Ground Speed / 60)
For example, you have to descent 10000ft with ROD 2000ft/min and you have ground speed of 300 knots. Dividing 10000 by 2000 will get you 5 minutes. If you divide 300 knots by 60, you get 5 NM/min. If you multiply 5x5, you get 25 NM - very simple indeed.
2. The rule of thumb for 3° glide slope is very simple, you multiply difference in altitude (let's say from present altitude to runway elevation) with 3 and you get distance.
Distance [NM] = Altitude Difference [ft] x 3
So if you are 4000ft above the runway, you should be 12 NM from the threshold. If you're closer, you're coming high and if you're further away, you're shallowing
