Assuming you are doing an attic, your batts should extend fully to lay over the top plate of the wall but serve no benefit inside the actual soffit. Your soffit should be allowed to vent upward through the attic. You may require a liner on the underside of the roof between every other or 3rd rafter and above the insulation to provide proper flow of air upward and out the roof top vents.
go to buildingscience.com for info
If by baffles you mean the foam strips that are designed to be placed against the roof sheathing between the joists to allow air flow above the insulation, then yes, slide these into place and your fine.
Shy of the top plate will give you cold corners at the ceiling, depending
on your location you could develop frost, etc
Lap the top plate & use the baffles for best results. Worst case I have heard of HI R value rigid foam as a block below the baffles...on a sharp pitched roof
I put a lot of thought (and took a lot of advice here) into blocking off my soffit (but still allowing for air movement of course) because of the fact that I have a raised hip roof. I think this ended up being a great solution for me. I bought a bundle of those foam boards (Z fold I think they call it?), cut them down to 24" pcs, took a razor and made 'wings' to staple to the 2x4, then ran a torch across at a pre-measured distance and put a fold in it, then installed. Now that the drywall is up, I will go back in and fill with blown in insulation.
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