300kts + 20kts tail - 180kts = 140kts to lose to intercept glideslope at 180kts flap 5. 140kts @ 10kts per nm = 14nm for deceleration.
40nm - 14 = 26nm for descent.
26nm x 3 = 7800' agl for a smooth descent on the safe side. anything faster or higher you may need to bring gear down early on the g/s.
regards