Knee Wall bottom plate question

Hello all,
I am finshing off the upper story of my 1 1/2 story Cape Cod, using the usual knee walls.
I am building them with a 2x4 bottom plate, 2x4 studs, and a 2x6 top plate bevel-ripped to the roof angle.
Here's my question(s):
Is the 2x6 top plate overkill?
Should the bottom plate be fastened to the top of the subfloor, or should sub-floor be removed in the area where the bottom plate will rest?
I think I saw an episode of Hometime where they removed a portion of subfloor in order to set the knee wall, but for the life of me I can't remember why.
Seems to me it would cause problems nailing off the bottom of the drywall later.....
I appreciate your opinions.
Thanks
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The 2x6 isn't necessary. You could build the whole thing out of 2x3s and use metal track top and bottom. There's no reason to remove the subfloor. You are not building a bearing wall - you're building a partition. Don't build the kneewall in a way that roof loads are transfered to the floor joists. Google and you'll find some details.
R
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