The dynamic load rating of 225 lb is the load the actuator can apply safely without damaging itself, so as long as the voltage is high enough for the motor to produce at least 225 lb, the dynamic load rating will be 225 lb. At 12 V, the actuator would be strong enough to lift far more than 225 lb, were it not for the risk of things breaking (you can tell this based on how fast it can still move at this load), so it should be able to generate a force of 225 lb for voltages well under 12 V. As I said previously, as you drop the duty cycle from 100%, the speed at 225 lb will decrease until at some duty cycle, it reaches zero. You can just play around with it in your system until you figure out what duty cycle gives you the speed you want at 225 lb (or whatever your actual load is).
We have not characterized at what voltage the actuator will no longer be able to lift 225 lb; you would need to figure that out for yourself. In case it helps, my best guess based on how the speed depends on the load would put it around 4 V (or a 33% duty cycle with a 12 V supply).