For reasonable comparison of the relative power of different cartridges, use the standard physics equation for the energy of a projectile: E = 1/2 mv^2, where E is kinetic energy, m is the mass of the projectile and v is its velocity.
For instance, to compare the 7.62x39 with the .308 (both of which are nominally 30 caliber), you need to know the weights of the projectiles and their muzzle velocities.
The standar 7.62x39 cartridge propels a 115-grain bullet at approximately 2300 fps at the muzzle.
A standard .308 cartridge propels a 148-grain bullet at a approximately 2800 fps at the muzzle.
Disregarding units, we can plug this ballistic data into the E = 1/2 mv^2 equation for relative powers of the rounds:
7.62x39: E = 1/2 x (115) x (2300) x (2300) = 304175000
.308: E = 1/2 x (148) x (2800) x (2800) = 580160000
Dividing 304175000 by 580160000 = .524, meaning that the 7.62x39 round is 52% as powerful as the .308 at the muzzle.