Math equation for variance

[kgallowaypa]

So I had this random thought the other day while I cam camping Verina Tomb. That specific mob spawns between a random 2-7 day variance. I started thinking if you could mathematically calculate the probability for any mob to spawn within their window but I couldn't create the formula on my own. So here's a thought experiment [You must be logged in to view images. Log in or Register.]

Lets say a given mob's variance is 16 hours for the pop to occur.
Evenly distributed each hour in the window comes out to a 6.25% chance each hour that the mob could pop.

It is known that if the mob does not spawn by the 15th hour, there is a 100% chance the mob will spawn in the 16th hour...but how would you calculate each change mathematically as the hours progress?

For example, A mob's window is open and 3 hours pass but no pop or event has occurred. I am guessing you just take the remaining hours left and divide by 1. So in this example, on the 3rd hour of the window, the next hour becomes 7.69% , 8.33% on the 4th hour, etc.

Is my math behind this accurate? There is no way you can predict the random variable of when the mob occurs, but this might be a safe bet. Any opinions?