I had to figure out what my fillet radius needed to be
based on the type fiberglass to be used to cover the joint.
So I took a section of 'glass and placed it in a hard 90° bend,
and let it form it's own radius and measured the result.
3/4 inch radius to produce a smooth fiberglass layup with
6 ounce tooling cloth. Then from the top of a plastic jar
made a tool for applying a smooth fillet and overspread cleanup.