Just saw this. I'd assume most assign probabilities to inside view qualitatively. So when combined with base rates, you'd say there is a 45% chance of this happening. But variable X, Y, Z change the percentage in my view by 10% to 55%. My preference would be to get more detailed on the base rate. For example, how does the base rate change for cases in the sample with variable X. But sometimes the sample isn't large enough for that sort of refinement. And I always like to check how sensitive my forecast is to changing probabilities.
I think the Base Rate book isn't supposed to be a new investing methodology. I think it is more supposed to be a helpful way to keep multi-year forecasts in check, by understanding the distribution of differing variables over time. I think the purpose is very mechanical in nature; more focused on improving forecasting than improving the thinking behind investing.