Main calculation : 3 times height to lose plus 5 miles to slow and configure
i.e 20000ft to 5000ft = 15 x 3 = 45 miles + 5 = 50 miles.
Rate of descent required to be precisely on descend plan is 5 x ground speed
So, let say we have 250kts GS (ground speed) : 5 x 250 = 1250ft per minute
Picture refer’s & belongs to Budi Mulyanto