I am at the end of a wall (finishing basement) and am doing a 90 degree corner with a rough opening for a door . Therefore my floor plate length from the ist wall to the end of the rough opening is only going to be the width of the 2 2x4's (jack stud and king stud-3"total or the most I can go would be the width of three vertical 2x4"s (4.5"). Rather than trying to do an actual 3" or 4.5" length PT floor plate, can I just use two PT 2x4's vertically all the way from the top plate to the concrete floor w/o an actual floor plate? These will be nailed to the adjacent wall & the ends would be glued to the floor.
Sure you can. But what I usually do in a case like that is use thinner treated wood (1x) or even a cut of treated plywood as my plate. That will accept hardened nails pretty well without splitting, and you can toenail your studs to that..
In truth, you only need to protect the untreated wood from direct contact with the concrete. Ofttimes, a piece of roofing paper under the wood will suffice. You see metal used all the time in the form of column base anchors, or truss plates (where the truss sits on the concrete lintel)
"True eloquence consists in saying all that is necessary, and only that which is."
François Duc de La Rochefoucauld
What Willie T said. The p.t. wood only need to be 2' long or so, with the second one 18" or so, just long enough to give you plenty of nailing room. Then the rest in doug. fir, or whatever. I usually don't nail a bottom plate if it's all studs, stacked.
Hint: make sure there is enough room for your elect. switch box on the side you need of the door. (That stud is movable, just so wallboard reaches). If wide trim casing, move blocking or stud now, for box installation later.
You knew that, see how great minds think alike! Be safe, GBAR