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I agree with Robert, there's no way to determine an actual number using this method. I do have a suggestion for a method that would be a lot more precise, and that I'm sure someone could devise a way to get actual pressure readings from.
For this method you'll want a road that's as straight and level as possible, and one where you can drive at a consistent speed.
Get a flat piece of wood (like plywood, say), about two feet high and about a foot wide. Also get a length of clear tube, I would say about four feet might suffice. Finding the right size of the inner diameter of the tube might require some experimentation, but I'm thinking maybe a half inch might do it. I imagine air tubing for a fish tank pump would be too narrow (but I don't know).
Fasten the tube in a U-shape onto the wood (without puncturing it), so that the upright portions of the tube are vertical and straight, and that the curved bottom part is not pinched enough to stop water from easily flowing through it.
Find somewhere in your vehicle to secure the piece of wood, where it is visible, so that it's vertical and doesn't move, with the open ends of the tube at the top. (you might also put it somewhere where the water accidentally spilling out won't ruin anything) Pour water into the tube (funnel will be handy) so that it's about half full.
Run that hose that's hooked up to the place in the engine bay where you're testing air pressure over to the clear tube, and connect it to the top of one of the open ends of the clear tube so that it's airtight. Leave the other end of the clear tube open.
Now, get in your car and start driving (without the plow at first, I'd recommend). This is where we experiment on if the clear tube is the right size or not. If the water levels don't move at all, or move so small a distance it would be difficult to measure changes (like, less than an inch), the clear tube is too big in inner diameter (or the connection's not airtight, or there's a clog in the hose, or the tube is pinched shut on the bottom). If water flows out the open end of the tube, or if you see bubbles passing through the water) then you need a clear tube with a larger inner diameter (or a smaller hole at the testing area).
Once you have the right size tube where the water will move a good amount at the speed you want to test (maybe about 3 to 6 inches), you can start comparing air pressures from different modifications. Mark where the water level is while the vehicle is stopped, and then mark where the water level is while at speed. You can compare the different water levels with and without the snowplow thing (and other mods as well), and this will give you a more precise comparison of the effects of different mods, even if you don't know the actual air pressure units. And if you supply the diameters of the hole, the hose, and the tube, as well as the water level changes, some enterprising individuals on here might even be able to calculate the actual pressure units.
This method could also be used to measure the pressure on the outside surfaces of the car, both high and low pressure areas. You could even do it without the holes in the car, just use a small enough hose at the location you want to test the pressure of, and make sure that the airflow isn't blowing directly into the hose. (I think pointing the test hose away from the wind would work best, if it's small the wake of it shouldn't mess with results too much, and at any rate it's located against the body of the car where airspeed is lowest, and pointing away would likely be more consistent than pointing it towards)
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