Or some strong tape along the line helps preserve the edge.
I've also scored the top face with a Stanley knife and straight edge to break the surface and give you a cut line. Like your idea though, can draw the line into the tape.
Thanks!
I'm buying them in white, and aiming to cut them with less than a 1/4" threshold between the walls on each side to allow for the walls not being square and any surface discrepancies. Thought when I get them as close as I can to perfect fit, will just finish the edges with a fine bead of caulk to neaten it up.
The walls were all over when I did mine. Couldn't get it to fit in the alcove as one walk bowed out in the middle slightly. Ended up putting a timber bead around the edge, but that was play for a desk.