There's probably a better alternative but here's one option:

=SUMPRODUCT(SUBTOTAL(3,OFFSET(A2:A251,ROW(A2:A251)-ROW(A2),0,1)),--(A2:A251="WINNER"))/SUMPRODUCT(SUBTOTAL(3,OFFSET(A2:A251,ROW(A2:A251)-ROW(A2),0,1)),--(LEN(A2:A251)>0))