I cannot imagine why anyone would want to know the sweep angle but.....If
you measure up the stays from the deck high enough to clear the house (if
there is one) and place a piece of tape. Measure from one spreader to the
other at the tape elevation. Then measure horizontally from the tape to the
mast. These figures define an isosceles triangle. Solve for the base angle
and that is the sweep angle. Sweep angle equals ....the angle whose cosine
is half the width measurement divided by the mast-to-stay measurement.