Do you mean run the paper perpendicular to the flooring? 45° would be diagonally
I never thought of that.
This stuff it pretty thin , we overlap 6” or so ( no tape).
We didn’t undercut the mortar.
Who ever re-faced the fireplace did it on top of the old plush carpet. I pointed out to the customer there was loose pieces of mortar on the carpet before we touched it.
Yeah thats old school NWFA way to run the felt, at a 45°. Unless of course your flooring is 45°.