I’ve got an email asking me about how to figure out the total bit revolution (RPM) at down hole when the mud motor is used in the drill string and the rotating at surface is also conducted.
There are two components contributing in the total revolution of downhole drilling bit using mud motor.
1. The rotor RPM of drilling mud motor– each drilling mud motor has its specific performance data which will tell you how many revolutions per flow rate flowing through it.
For example (see the image below), it shows Rev/Gal which means the rotor will turn 0.147 turn/min per one gallon/min of drilling mud passing thought.
2. The rotary speed from top drive or rotary table.
The total drilling bit revolution downhole:
The total drilling bit revolution is equal to summation of the rotor RPM at specific flow rate and the rotary speed on surface.
Please follow the example below to determine the bit revolution down hole.
The performance data of mud motor -
Rev/Gal = 0.147
Flow rate = 320 gpm
Rotary speed = 220 rpm
The rotor rpm based on 320 gpm = 0.147 x 320 = 47 rpm
Total bit revolution down hole = 47 + 220 = 267 revolution per minute (rpm)
Ref book: 
Formulas and Calculations for Drilling, Production and Workover, Second Edition