Cutting quadrant beading

I have removed all of the silicone around the bath and now I am trying to fit quadrant beading. I have one right-angled corner that needs two pieces of beading to go around. I thought that I would use my coving mitre box to do this as it has two 45 degree angles on it, which is what I thought I had to cut two of to then join together. However, after numerous cuts, the two pieces are not slotting together properly (triangular gap at the bottom).

I'm not sure if the issue is because of the shape of the quadrant beading which has to be fitted at an 'angle' to the wall (not lying flat down). If I place the two pieces down flat on the carpet, they join up and fit together, but the issue is when I place them where they should be on the side of the bath/tiles, then a gap appears at the bottom. Any suggestions on how to get around this would be most welcome. Thank you.