If you consider that it takes about 1 to 1 1/2 weeks to get from nearly all cloudy to your desired amber %, and that your flush period should be about 1-2 weeks, then it is actually a useful way to gauge your flush time without actually knowing the exact date on which the crop is due to come down. So yeah, it will save you the 'it's not even close!!!' headaches.
I think at the end of it all, the presence or absence of calmag in the watering solution is less important than during growth- maybe cut the ferts first, then cut the calmag later, in the last week. I don't know what would be perfect, but that would be logical at least.