WoW:USERAPI GetUIParentAnchor

Returns SetPoint args for the frame, anchor is relative to the nearest corner of the screen.

local function GetUIParentAnchor(frame)
	local w, h, x, y = UIParent:GetWidth(), UIParent:GetHeight(), frame:GetCenter()
	local hhalf, vhalf = (x > w/2) and "RIGHT" or "LEFT", (y > h/2) and "TOP" or "BOTTOM"
	local dx = hhalf == "RIGHT" and math.floor(frame:GetRight() + 0.5) - w or math.floor(frame:GetLeft() + 0.5)
	local dy = vhalf == "TOP" and math.floor(frame:GetTop() + 0.5) - h or math.floor(frame:GetBottom() + 0.5)

	return vhalf..hhalf, dx, dy
end