We consider photometric reverberation mapping, where the nuclear continuum variations are monitored via a broad-band filter and the echo of emission line clouds of the broad-line region (BLR) is measured with a suitable narrow-band (NB) filter. We investigate how an incomplete emission-line coverage by the NB filter influences the determination of the BLR size. This includes two basic cases: 1) a symmetric cut of the blue and red part of the line wings; and 2) the filter positioned asymmetrically to the line centre so that essentially a complete half of the emission line is contained in the NB filter. Under the assumption that the BLR is dominated by circular Keplerian orbits, we find that symmetric cutting of line wings may lead to overestimating the BLR size by less than 5%. The case of asymmetric half-line coverage, similar as for our data of the Seyfert 1 galaxy 3C 120, yields a BLR size with a bias of less than 1%. Our results suggest that any BLR size bias due to a narrow-band line cut in photometric reverberation mapping is small and in most cases negligible. We used well-sampled photometric reverberation mapping light curves with sharp variation features in both the continuum and the H beta light curves to determine the geometry type of the H beta BLR for 3C 120. Modelling of the light curve, under the assumption that the BLR is essentially virialised, argues against a spherical geometry and favours a nearly face-on disk-like geometry with an inclination i = 10 degrees +/- 4 degrees and an extension from 22 to 28 light days. The low inclination may lead to a larger black hole mass M-BH than that derived when using the average geometry scaling factor f = 5.5. We discuss deviations of Seyfert 1 galaxies from the M-BH-sigma(*) relation.