A holographic superconductor whose scalar mass depends on the gauge field strength,
M(\Fsq), reproduces a superconducting dome for a suitable
M, and recovering that
M from a given dome has so far taken days for a single training run. We propose a new way of training this model, with which an inversion takes from about ten minutes to an hour. Training needs the gradient of the condition that fixes the critical temperature, which the earlier method obtains by finite differences, repeating the bulk integrations for every training parameter. Here that condition is obtained, without any fit, from two integrations started at the horizon and at the boundary, and its derivative with respect to
M is an integral over the same two solutions, so the gradient needs no integration of its own. We use the speed to study the part of
M that a dome cannot determine, on the interval between the value
\Fsq takes at the horizon for the lowest doping and
\Fsq=0, at which
M is the scalar mass
M(0) that fixes the dimension of the dual operator. We hold the scalar mass at several values, which we call pinned masses, retrain everything else at each, and find that the reconstructions agree wherever the horizons of the dome reach, including the minima of
M, and differ only on that interval. A rule that keeps the reconstruction with the simplest closed form recovers both the scalar mass and the mass function of a test dome. On Gaussian and double-Gaussian domes and on the measured phase diagrams of YBa
2Cu
3O
y and 2M-WS
2, however, the pinned mass it keeps rests on ties or on narrow margins, so for these targets the scalar mass is left open. The dome thus constrains
M where its horizons reach, and fixing the dimension of the dual operator needs a second observable.