StratfordOne:
Lemme explain attic ventilation first, and then I'd tell you what I'd do:
Some people down in the southern states figure attic ventilation is to allow the hot humid air out of the attic in the summer to make the house cooler and more comfortable, but hot humid air isn't going to damage your house, and so that's not why it's necessary.
Where I live (about 100 miles north of North Dakota's northern border) attic ventilation is mostly needed to eliminate the warm moist air that inevitably rises and finds it's way into the attic in the WINTER.
When that warm moist air gets into the attic, it condenses or freezes to the coldest surfaces in that attic; which is generally the underside of the roof and the roof rafters. That frost or condensation doesn't do any harm... until spring when it melts and drips onto the ceiling insulation, getting the ceiling insulation wet.
Insulation works by keeping air stagnant, so wet insulation takes forever to dry out, and having wet insulation in contact with wood ceiling joists is a prescription for wood rot.
And, on top of the wet insulation being in contact with the wood, you'll typically have a plastic vapour barrier under the ceiling joists that prevent the moisture from getting the plaster ceiling wet, and that lulls the homeowner into thinking there's no real problem until the damage is severe enough that the ceiling starts to sag. I myself have seen a photo of one rented house that was turned into a marijuana grow-op that had 3 inches of frost on the attic roof and rafters.
By providing some holes in the attic where air can get in and out of the attic, but not snow or rain, then the wind will ventilate the attic, thereby eliminating that moisture that could potentially cause an awful lot of damage.
THEORETICALLY, soffit vents on both sides of the attic to let cold air, and a ridge vent on the top of the roof works the best because it uses the inevitable heat loss from the house to drive a convective current that eliminates the humidity from the attic. However, what normally happens in reality is that days with no wind are comparativelyh rare and virtually any wind will break up that convective current that soffit/ridge vents rely on for theoretical 100% sweep efficiency. What normally happens is that there's some wind, and the wind blows cold air into the soffit vents on one side of the attic and the warmer moister air is pushed out the soffit vents on the other side of the attic. But, as long as the moisture is eliminated from that attic, that's all that matters, and a cross wind will do that as effectively as a convective air current.
If it wuz me, instead of using plywood, I would use 1X6 construction grade lumber boards to make your floor, and leave a 1/8 inch gap between each board. That would allow for ventilation of the ceiling joists and the wood floor.
Now, the reason why 1X6 boards are commonly nailed down at a 45 degree angle to the joists instead of perpendicular to them is that by nailing the boards down at an angle you increase the rigidity of the house from twisting in a strong wind, and that reduces the liklihood of plaster cracks in the weak areas, like at the top corners of doorways.
Also, in most houses, access to the attic is through a small entryway, thereby precluding you're being able to get 4X8 sheets into the attic. Shorter boards nailed or screwed down in a staggered arrangement would solve that problem.
And, to confirm you have enough attic ventilation now, go up into your attic on the coldest day (or night) of the winter. Look for frost or condensation on the underside of the roof or dirty spots on the ceiling insulation (because that dirt can be caused by water running down the rafters and dripping off a knot or something onto the insulation directly below).
If you don't see any frost or condensation, then your attic is well ventilated and you don't need any more ventilation up there.
Hope this helps.