Tv(b-v) transformation coefficient for Baader Johnson V + ZWO ASI120MM-S

Even having a single photometric filter (Johnson V), it is useful to determine Tv(b-v) transformation coefficient. This allows the transformation of measured V magnitudes if B-V color indices for target variable and comparison stars are known (assuming that B-V index for the variable does not change significantly).
One of the recommended AAVSO's star standard star fields is an open cluster Messier 67.
Observations of M67 were carried out on 25 Mar 2020 using ZWO ASI120MM-S uncooled CMOS camera with Baared Johnson V filter attached. Sky Watcher 150/750 Newtonian was used as an imaging telescope.
500 images with an exposure of 12s were collected. They were calibrated, as usual, aligned by stars and stacked to 5 stacks of 100 frames each.
Measurements were done using AstroImageJ software. 29 stars were measured, results were averaged by 5 stacks.
The resulting V-v on B-V dependency is shown in Fig. 1 (where V is a catalog magnitude, v is an instrumental magnitude, B-V is a color index).

Fig.1

Tv(b-v) transformation coefficient, determined for the slope of an approximation line, turned out to be equal 0.0086+-0.0042. It is rather small, so even untransformed observations with this filter should give reasonable values.