The big thing here is to make sure that the pin is rigid to the blade and is
at 90deg. to the blade.
Normally I put a piece of scrap plywood on top of the table and clamp it to
the table - a few bolts in appropiate holes or a clamp to the bottom of the
table with aligning pads is a nice way.
Large radii tend to be a bit harder and require that you put legs on the far
end of the plywood to support the pin end of the plywood.
The pin can be anything from a real pin of some diameter to a nail or other
pointed thing to locate the center point. With the nail method, you have to
be a bit careful that the center doesn't move about as you do your cutting.
The process of cutting curved surfaces is an established process in
woodworking.

--
Bob May
Losing weight is easy! If you ever want to lose weight, eat and drink less.
Works evevery time it is tried!