To find the distance from the airport that you should start a descent

• subtract the pattern altitude (MSL) from your current altitude,
• delete the three 0s (dividing by 1000)
• then multiple by 3 to give you the distance you should start you descent to give you a nice steady 500 fpm decent.

Current Altitude: 6500, pattern 1500'. 6500-1500/1000*3 = 15 miles out.