Actually, I am getting very indeterministic results...
As I am trying to find the right offset and size it is sometimes drawn on top and sometimes drawn below the NineSlice border...
Code:
myFrame.NineSlice.TopRightCorner:SetDrawLayer("BORDER")
myFrame.exportButton.btnDivLeft = myFrame.NineSlice:CreateTexture("cosFix_btnDivLeft", "BORDER")
myFrame.exportButton.btnDivLeft:SetPoint("RIGHT", myFrame.exportButton, "LEFT", 5, 0)
myFrame.exportButton.btnDivLeft:SetAtlas("UI-Frame-BtnDivLeft", true)
local oldX, oldY = myFrame.exportButton.btnDivLeft:GetSize()
local shrinkFactor = 0.854
myFrame.exportButton.btnDivLeft:SetSize(oldX*shrinkFactor, oldY*shrinkFactor)