P0 = r0B0cKk´
where P0 is the central momentum in units of eV/c, r0 is the design bend radius of the spectrometer (r0 = 8.4 m), B0 is the central magnetic field in the spectrometer in tesla, c is the speed of light in ms-1, K is a correction factor for fringing fields in the dipole (K = 1.00708), and k´ is a correction factor for a known misalignment of the Q3 quadrupole (k´ = 0.999833). The uncertainty on the absolute value of P0 is estimated to be plus or minus 2 x 10-3 of the value of P0. An expression is also given for the determination of B0 based on the field read in the "low field" NMR probe.