  We use the observed jet boundary transition from parabolic to conical shape,
which was earlier discovered as possibly a common effect in active galactic
nuclei, to estimate a black hole, a jet and an ambient medium parameters. We
explained earlier the geometry transition as a consequence of a change in the
jet inner properties: a transition from a magnetically dominated to an
equipartition regime. This interpretation allows us to estimate a black hole
spin, a black hole mass and an ambient pressure amplitude, using the observed
jet shape break position and the jet width at the transition point for 11
active galactic nuclei. The black hole spin values obtained using our method
are consistent with the lower estimates for the sources with redshift $z<2$
from the spin evolution modelling. We find that the method of a black hole mass
determination based on the relation between the broad-line region size and its
luminosity may underestimate masses of the sources with large jet viewing
angles. We propose a new method for the black hole mass determination, with the
obtained masses being in interval $10^8-10^{10}\;M_{\odot}$. The range of the
values of the ambient pressure amplitude points to the uniform medium
conditions for the sources in our sample, with a tentative indication of higher
pressure around FRII sources.
