Draw your first arc on what will be the top of your rail. You should only have to trace this once for the outside. If your jigsaw can handle it, cut both boards at once.
This first arc will be the overall size of your table. If you plan on down-sizing your table, do so here. You’ll be using this outside arc for the measurements for the rest of the table.
To draw the arc, I made a makeshift compass. Basically, find the center of the board near one end. Nail or screw it in (don’t worry, we’re cutting the middle out and discarding it anyway). Use a pencil to draw the arc.