The most precise measurement of total magnetic field strength is provided nowadays by quantum magnetometers based on the Zeeman effect in radio-spectra of nuclei and atoms. The optically pumped potassium magnetometer based on a single resolved magnetic resonance line is considered to be the most advanced one when the highest base line stability or/and short-term sensitivity are required. The ultimate potential resolution of potassium magnetic resonance line has been found better than 10 fT/Hz^1/2 under expected base-line stability not worse than 10 pT. This highest performance can be reached using paraffin coated cells of about 1 litre volume under very low pumping light intensity and potassium vapour density providing the resonance line width of about 1 Hz. The base line stability can be further 10 times improved at the expense of short-term sensitivity. Both lamp pumping and laser pumping regimes have been studied. The laser pumping further improves 2-3 times both leading characteristics.