Dimer mole fractions beta in a potassium beam were measured for a wide range of stagnation parameters. A comparison with a theoretical fitting formula was made. Very good results were obtained for the latter using as the stagnation temperature an effective stagnation temperature which includes a contribution from the heat released due to dimer formation in the gas expansion. This interpretation of the stagnation temperature allows a convenient description of dimerization processes for species with high binding energy and heat of formation, further an easy comparison with those species, which binding energy is low. For the theoretical formula we are able to give an assumption-free scaling constant for potassium.