Configuration                                  K_t        error
Annulus, P2 curved, finest mesh (21,220 dof)   2.99981    0.0063 %
Annulus, extrapolated to h = 0                 3.00002    0.0005 %
Finite plate, extrapolated                     2.99970    0.0101 %
Conservative Richardson variant                2.99937    0.0211 %