For maximum accuracy, the ring diameter should be extrapolated as in figure 3 in order to compensate for both the curvature and thickness of the glass envelope; the lower the anode voltage, the larger the ring diameter, and the greater the influence of these parameters.
This takes a little bit of geometry. If we say that R is the radius of curvature and that t is the bulb thickness and you make measurements of the diameter of the rings D as they appear on the inside surface of the bulb then the extrapolated De will be