This is a method that boat builders use to make large arches without using gigantic compasses or trammels.
It is really rather easy to do and quite accurate.
You do not need a center point.
Let's take a practical guitar building example -
For a 24 inch radius dish with a 25 foot radius, the run would be 12" and the rise would be .240".
For this example you'd need to made the jig taller to make it stiff enough. Just add another inch or 2 to the bottom of the jig while maintaining the shape of the upper portion.
Drive nails into the the template at the center and the ends of the arch.
Position the jig with the hypotenuse against the end nail and the flat against the center nail.
Hold a pencil in the notch and advance the fig while bearing against the nails.
Flip the jig to complete the other side.
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