The 10-probe NMR system for measuring the polarization in the deuterated butanol target of the Spin Muon Collaboration is presented. It is calibrated by determining the integrated spectrum in thermal equilibrium at 1 K. Thermal stabilization and control of electromagnetic interference were effective in reducing drift and electronic noise, so that high quality thermal equilibrium signals were obtained. The calibration constant was determined with a reproducibility of 2%. Systematic effects of circuit drift and of the magnetic field shift have been studied. The resulting 4.4% uncertainty of the calibration dominates the 5% overall error on the target polarization value which was typically 0.45.