First how many 1000s of feet you do you need to lose? mulitply that 1000 number by 3 and that is how many miles you should start your gradual descent.
- or example you are at 6500 feet and need to descend to pattern alt 1500 feet you need to loose 5000 feet mulitply 5 x 3 = 15 miles out you need to descend
At what rate should you descend?
Take half your groundspeed and mulitply by 10.
- you are flying at 100kts, half is 50 x 10 = 500fpm