in your house are not exact 90 degrees.
With this method you can cut or saw the right corner.
Step 1: What you need
A piece of Letter or A4 format (used) paper with one straight edge.
A ruler or other thing with parallel edges.
if the piece of paper is to big fold in two.
Step 2: Arange the paper
of the wall
Step 3: Draw a line
Step 4: Fold the paper
Please take a look at the picture.
(one picture tells more then 1000 words)
Step 5: Draw the half corner on the base shoe
is against the edge of the base shoe now you can draw the diagonal (half the corner)
on the base shoe.
Cut or saw on the line.