The general rule for correcting a high blood glucose needs a target number to start. I'm guessing from what you wrote is that your target is 120 mg/dL.
The first step is to calculate the amount of the correction you need:
216 - 120 = 96
Then use your insulin sensitivity or correction factor (yours is 25) and divide that into the number calculated above:
96/25 = 3.84 units of insulin
If you're using a syringe and vial, I would round up to 4 units but if you want to add a margin of safety then deliver 3.5 units.
I'm a little confused about what "1 unit for 40 over 120" means. Perhaps your doctor wants to be more conservative with corrections that are near your 120 goal. If that's true then your doctor may want you to take a less aggressive correction, but it does confuse the situation. If you can clarify, I might be able to check your math.