I just wanted to clarify the setup I use for setting internal lathe tools at centre height using the tangent of the stock and a steel rule.
I simply move the tool to the opposite side of the workpiece and set the boring tool on the outer diameter, in the same way as the external tools. If using a boring bar, it is highly unlikely that you would be using a centre to support the work piece. The photo below shows the setup for internal tools.
Thank you again for viewing
The Home Engineer