The circuit will PWM beyond 25 kHz as long as the solenoid doesn't have to follow it. I guess the objective is to induce a current from 0 up to 2.4A (12V/5ohms) into the coil, and have the coil do the smoothing. Should work.
To get the "exact" frequency, you need the inductance of the coil, the mass of the solenoid plunger, and the spring constant of the return spring (or likely in this case the hydraulic pressure). At higher frequencies, eddy currents in the solenoid frame and plunger are noticeable. Using all of the constants, variables, and parameters above, solve for the resonant frequencies and associated damping factors. Then decide if you want to operate at mechanical resonance, the electrical resonance, or more likely, to avoid both.
My guess is it's a highly damped system and the frequency doesn't matter, within reason. Try 100 Hz and if it chatters move up till it doesn't. If you get into kHz and it stars to sing, you may be higher than necessary.