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tokens: with any field characteristics obtained from circular polarization modeling. Considering our ZDI results, the mean vertical field corresponds to the surface-averaged unsigned radial field reported in Table 3 . Our = 98–168 G is a factor of 2.9–5.6 lower than the vertical surface fields suggested by MacDonald Mullan ( 2017b ). If, instead, we choose to compare their predictions with our measurements, the theoretically estimated field should be multiplied by assuming isotropic field vector orientation. This yields a total surface field strength of 850–950 G, which is 3.3–4.0 times lower than determined in our paper. To summarize, theoretical models by Feiden Chaboyer ( 2013 ) appear to predict a more realistic surface field strength for the components of YY Gem than the one found in the approach by MacDonald Mullan ( 2014 , 2017b ). However, this conclusion should be taken with caution given the ambiguity of relating the vertical surface field parameter of the latter theoretical calculations to observations. 7.3. Estimating Magnetic Flux from X-Ray Luminosity In the absence of direct constraints on the surface magnetic properties, many studies resort to an approximate estimate of the mean magnetic field strength. The X-ray emission is often considered to be a particularly useful magnetic proxy. Pevtsov et al. ( 2003 ) demonstrated that the total unsigned magnetic flux measured for solar magnetic regions at different spatial scales shows a power-law dependence on the X-ray spectral radiance. The few measurements reported at that time for active late-type stars fell on the extrapolation of this relation to higher magnetic flux values. Feiden Chaboyer ( 2013 ) recalibrated this relation using Stokes I field strength measurements of low-mass dwarfs. Both Feiden Chaboyer ( 2013 ) and MacDonald Mullan ( 2014 ) attempted to verify their theoretically predicted surface field strengths with indirect field strength estimates using the X-ray emission of YY Gem. This exercise was, however, hampered by the contamination of the ROSAT X-ray measurements of YY Gem (Voges et al. 1999 ) by the nearby early-type companions Castor A and B. More recent Chandra observations of YY Gem (Hussain et al. 2012 ) provided images with a high angular resolution, allowing one to separate the low-mass binary from the A-type stars. The combined quiescent X-ray luminosity of YY Gem is reported to be (3–3.5) × 10 29 erg s −1 . Assuming equal luminosities of the components and using their fundamental radii yields = 770–820 G with the calibration by Feiden Chaboyer ( 2013 ). This is significantly smaller than the observed obtained in our study, indicating that the magnetic flux versus X-ray luminosity relation may be appropriate only for large stellar samples but is unlikely to be precise enough to provide a meaningful field strength estimate for individual stars. Another problematic issue with applying this relation to active stars, apparently entirely overlooked in the