Are you sure that this test is EtG and not a standard alcohol assay? The sample, procedure and container, would be the same. Are the results available on-site or sent to a laboratory, how long does it take to get results?
EtG testing is, to the best of my knowledge, used exclusively in settings where total abstinence is required: rehab, court programs, licensing boards in conjunction with provisional license restriction, etc. Except in the TA setting the test has no value.
EtG independently, for instance, could not determine if you were drunk at the time the test was taken. That requires a Blood Alcohol read. EtG can only determine that you have had (assumed consumption) alcohol in your system at some time in the past and in unknown quantity.
The purported purpose of EtG is to detect drinking after systemic alcohol has left the system.
Maybe this is all known to you but I find this practice reminiscent of the Temperance crusades of the 19th century.
As to your request about timing EtG, I really can't be any more clear about my inability to apply the research to a specific individual.
I can tell you in general terms that the key to EtG is moderation and spacing of drinks which also applies to ethanol metabolism. The elimination curves are almost identical offset by about 5 hours down to a very low level.
Elimination is an ongoing process (not cumulative), so when you have one drink it will be absorbed, distributed and eliminated in about one hour*, during distribution the BAC will peak at around .02. If you begin a second drink at the end of that hour your BAC will not register .04 (cumulative) but stay around the .02 range because first drink has been metabolized.
EtG is created in the final step, Phase II, of metabolism but as soon as it is synthesized it likewise begins elimination (EtG/EtS are not cumulative).
The amount of ethanol in your system at the time of final drink is key as opposed to the total number of drinks consumed. The controlling factor is time...but when it comes to EtG we don't know how much time and we don't know how much EtG is synthesized by any given individual.
*This is a commonly used estimate. In fact this time may vary by 350% by gender and drinking history alone.