A detailed article on tuning the hurdy-gurdy’s tangenets using pure intervals by Ernic Kamerich:
“On a hurdy gurdy playing alone there is no harmony apart from the combination of melody and drone, so the obvious perfect way for tuning the tangents is by making all intervals pure between chanterelle and bourdon: a natural harmonic scale. That works well and makes the instrument sound sweet and beautiful.
However, most hurdy gurdies can use more than one bourdon and then things get more complicated. Moreover, playing together with other instruments may ask for a different tuning. Here will be explained what can be done without losing the sweet character of natural harmonic tuning.”
Much, much more here.