Alcohol metabolism is an ongoing process which begins with the first swallow. The total number of drinks consumed is less important than the rate of consumption. The liver metabolizes ethanol at a fixed rate and only when the rate of consumption exceeds the metabolic rate does ethanol accumulate in the circulatory system (blood alcohol content BAC) leading to impairment. So at the same time alcohol is being absorbed it is likewise being eliminated by the liver at the other end of the spectrum. By the time you consume your third drink the first drink has been eliminated (example based on normal rate of consumption)
There really is no precise method for calculating BAC. Individual metabolic rates vary greatly and then numerous factors influence the individual rate. One factor that is important but impossible to calculate (can only be assumed) is a history of heavy drinking. The liver has what I call "an overdrive system" called MEOS* that seems to develop following continued heavy drinking. MEOS increases metabolism by as much as 100%. So a social drinker will metabolize a drink in about an hour while a chronic drinker may metabolize the same amount in 25-30 minutes.
All of the BAC estimating schemes are based on population averages, (i.e. one drink per hour), and have only minimal benefit in individual cases. The foregoing involves the metabolism of ethanol.
EtG is a by-product of the metabolic process and has a longer biological life than the parent drug. The problem with calculating the half life of EtG is that it is related to an 1) unknown metabolic rate, 2) individuals produce EtG at varying rates and even the same individual may produce varying amounts from time to time. As a result the detection window for EtG is all over the board.
While ethanol metabolism has a very long history and voluminous research giving one more confidence in the averages, not so with EtG. Despite all of the outlandish claims there is very little supporting science. The small amount of research available is of very limited value, conducted on small groups often less than 20 subjects, has focused on males, unrepresentative groups (chronic alcoholic in detox), and failed to apply uniform criteria.
With all that being said, if you would like to calculate, in very broad terms, an ethanol elimination rate, here is a resource that includes more of the pertinent factors than most other calculators: Blood Alcohol Content Estimator from AODWiki
*Microsomal Ethanol Oxidizing System