Formula for calculating validator inactivity penalties?

Hey all, I'm trying to create an excel model that calculates a validator's estimated slashing penalties, based on variables such as Number of validators currently online and Number of validators slashed in 36d period.

I have already put together formulas for two of the three slashing penalties: initial penalty is simply 1/32 of stake, and midpoint correlation penalty is midpoint balance * Num of slashed validators/Num of active validators, rounded down to nearest 1ETH. Both of these formulas are simply explained here.

However, that document doesn't give a detailed formula for inactivity penalties… the best we get is:

"further penalty of up to 8192*((14+26​)/64)*32b = 82,739,200 Gwei = 0.0827 ETH, based on 500k validators, where b is the base reward per increment. This assumes that the chain is not in an inactivity leak; the penalties will be much higher if it is."

Obviously we are talking about a fairly small amount of ETH here, but I'd like to do this right and calculate the penalty myself, rather than just using the example of 0.0827 ETH @ 500k validators. Does anyone know the exact formula for calculating inactivity penalty, and how it changes when there is an inactivity leak?

Thanks in advance.