I have not used the Radar Mile method before but doing a GOOGLE search revealed this.
Radar timing is usually expressed in microseconds. To relate radar timing to distances traveled by radar energy, you should know that radiated energy from a radar set travels at approximately 984 feet per microsecond. With the knowledge that a nautical mile is approximately 6,080 feet, we can figure the approximate time required for radar energy to travel one nautical mile using the following calculation:
A pulse-type radar set transmits a short burst of electromagnetic energy. Target range is determined by measuring elapsed time while the pulse travels to and returns from the target. Because two-way travel is involved, a total time of 12.36 microseconds per nautical mile will elapse between the start of the pulse from the antenna and its return to the antenna from a target.
This 12.36 microsecond time interval is sometimes referred to as a RADAR MILE, RADAR NAUTICAL MILE, or NAUTICAL RADAR MILE.
1 Radar Kilometer = 2 · 1000 m = 6.66 µs (1)
3 · 108 m/s
1 Radar Mile = 2 · 1852 m = 12.35 µs (2)
3 · 108 m/s
The range in kilometers to an object can be found by measuring the elapsed time during a round trip of a radar pulse and dividing this quantity by 6.66.
The range in nautical miles to an object can be found by measuring the elapsed time during a round trip of a radar pulse and dividing this quantity by 12.36.
If we use PRT = the Pulse Repetition Time
PRT = 1 000 000 micro seconds / PRF
Range = PRT / 12.36 from GOOGLE extract
PRT / Range x 12.36
For 39 NM range
30 NM x 12.36 = 370.8
PRF = 1000000 microseconds / PRT = 1000000 / 370.8 = 2696
Which is approximately 2700.
For 50 NM range
50 NM x 12.36 = 618
PRF = 1000000 microseconds / PRT = 1000000 / 618 = 1618
Which is approximately 1620