What the premium is
When you sell an option you collect the premium up front and take on the risk of a large move against you. The variance risk premium is the gap between the variance option prices imply and the variance that actually follows (Carr & Wu, 2009). Research has found it informative about future returns (Bollerslev, Tauchen & Zhou, 2009).
A common shortcut compares today's implied volatility with recent realized volatility. It is useful context, but the two cover different windows: one looks forward and the other looks back.
An example
A stock's one-month implied volatility is 30% and its realized volatility over the last month was 20%. The spread looks generous. If earnings are due next week, though, the 30% may be a fair price for that event, and the stock could move more than the option priced. The spread describes the past month and today's price; whether selling pays off depends on what happens next.
Scores and sizing
Volatility spreads, term structure, skew and the gamma regime can all feed a view on whether selling premium looks well paid. A score built from them summarizes those inputs. It is not an expected return or a win rate.
Caveat: the Kelly criterion gives a growth-maximizing bet size only when you know the probabilities and payoffs. Option sellers rarely know either precisely, and the losses come in rare, large jumps. Using a fraction of Kelly reduces risk but cannot fix wrong inputs.