Your procedure gives a de novo probability, but I don’t quite see how to combine it with any preexisting information you might have.
I mentioned it was a uniform distribution, where each box is equal. If you had pre-existing information that December and February will be absolutely critical while January is less important, then you can move on to more complex versions. For example, here’s a calculation with a normal distribution instead: http://lesswrong.com/lw/5hq/an_inflection_point_for_probability_estimates_of/4291
I mentioned it was a uniform distribution, where each box is equal. If you had pre-existing information that December and February will be absolutely critical while January is less important, then you can move on to more complex versions. For example, here’s a calculation with a normal distribution instead: http://lesswrong.com/lw/5hq/an_inflection_point_for_probability_estimates_of/4291