I have no first hand experience of the work of Brian Maber. Even if I did, I am not sure I would want to express an opinion on how reputable he is. I think this is something best left to his readers to judge for themselves.
As for the question of whether the Coppock would give a signal at a certain value on a certain date, this is a simple "what-if" question. If the value of the Coppock indicator is negative at the end of the month, we know that a signal is given if the Coppock value is less negative by one or more than the value the previous month. If you want to watch during the month how close the Coppock indicator is to giving a signal, it would be reasonably easy to work the calculations backward to obtain the minimum index value that would yield a signal. This is undoubtedly all Mr Maber has done. Of more interest might be to know what method he is using to calculate the Coppock indicator, because on my spreadsheet the Coppock is not yet in negative territory, so is incapable of giving a signal.