  Coulomb-nuclear interference in the single transverse spin asymmetry A_N is
often considered as a possible absolute polarimeter for proton beams. The main
uncertainty in this is the unknown hadronic spin-flip amplitude. This
uncertainty is analyzed here in the context of the challenge of a 5%
polarization measurement at RHIC. Possible constraints on the spin-flip
amplitude from measurements of the differential cross-section and the double
transverse spin asymmetry A_{NN} are discussed.
