It is not "considered" a flat vent, because it IS a flat vent. The Y/45 is the approved way. You have to install the Y low enough so that the vertical pipe gives you the proper clearance for the toilet.
Although most I see are just a couple feet away from the stack.
And would that be a vent above the santee?
A santee and trap arm is good for six feet on a toilet, assuming that the pipe above the santee is a vent.
No waste can run down the vertical stack from above unless the toilet is vented before it hits the stack.
A santee and trap arm is good for six feet on a toilet