Proper Flyback Current Protection Circuit for Variable Voltage Solenoid Driver
You don't need change anything from default flyback protection. It is used to protect circuit from huge voltage spike that is induced by the coil. Inductors don't like fast current changes, generally speaking:

$$V = L\dfrac{di}{dt}$$

So, when the coil switch occurs, voltage in midpoint goes up to huge value. In order to avoid that, you need to add a path, in which current will be suppressed (snubber). Diode to the left is doing all the job, and diode to the right is useless (as voltage in midpoint is negative).

However, your circuit will change speed in which magnetic field change vill occur, but not the maximum magnetic flux. I would recommend you to check current controled coil instead of voltage controled, as with that you could limit the actual strength of a field. Sample circuit:







simulate this circuit &ndash; Schematic created using CircuitLab