Math was never my strong point. Average skills are all that is required. As an example of the typical math involved:
ATC issues a clearance to cross a fix at 11,000 ft and a speed of 250 knots. You are flying at 31,000 ft (20,000 ft to decend) and your speed in the decent will be cruise mach until transitioning to 300 knots indicated (50 knots to slow). Decending at idle power and 3000 feet per min.
Thousands of feet to decend, multiplied by 3, plus the distance to slow to 250 (20 times 3 plus 5 = 65 miles) The DC9 I fly will require about 1 mile to slow 10 knots with the power at idle in level flight at a fairly low altitude. To slow 50 knots requires aprox 5 miles.
This is just a rule of thumb. During the decent you should be re-computing to assure that you will cross the fix at 11,000 and 250 knots. Strong tailwinds/headwinds will require additional adjustments. You may want to wait till 55-60 miles with a strong headwind or start the decent at 70-75 miles with a tailwind.
This is just an example of a rule of thumb used every flight.