The authors report new measurements, by Faraday rotation spectroscopy, of relative f-values and multiplet splittings for high Rydberg states in the potassium spectrum. They show that, in general, it is necessary to take full account of the gradual transition from Zeeman to Paschen-Back coupling, via intermediate coupling conditions, in order to represent magneto-optical spectra correctly. When this is done, very good agreement between theory and experiment can be achieved over the full extent of the MOR profiles, and much useful information can be extracted, even for very high Rydberg states. The results are compared with recent theoretical predictions.