Solenoids have a distinct set of characteristics that dictate that the amount of current required to pull them in is MUCH greater than that needed to hold them in. This is because of the magnetic circuit that closes the air-gap reduces, and in some models closes, when the actuator moves in.
As such, many solenoids are rated at an actuation voltage and a continuous holding current.
It would appear your solenoid requires 12V to get you the initial indicated pull force. This will result in a much larger current than the 400mA needed to hold the solenoid pulled back. Based on your figures, the coil is indeed ~5.4ohms at DC.
As such, you need to initially activate the solenoid with the full current for a brief period and then, assuming you want the solenoid to hold there, cut back on the current.
The current reduction can be accomplished by using two different drivers, one high and one low current.
A better alternative is to switch from the DC activation signal to a PWM controlled hold signal. The latter should be at a high enough frequency to maintain the lower constant current in the inductance of the coil without saturation or complete decay. Since the device data information is sparse at best, you may need to experiment a bit to determine the inductance when the actuator is pulled back. This value will be somewhat greater than when it is in the rest position.
The fly-back circuit should be designed to accommodate the holding current continuously and the peak-current sporadically.
The issue with this kind of circuit though is a failure in the control system or the MOSFET will result in full current flowing through the coil continuously which would cause the latter to overheat. As such, some additional protection is warranted.
