While many folks do as OGM does, choosing a multi flock ration, I usually feed layer. UNLESS I'm in the middle of a deep freeze (temps single to minus digits every night, highs of minus digits to/low 20's during the day for days on end) and UNLESS I have a crop of chicks, pullets/cockrels, or if birds in heavy molt. If dealing with any of those situations, I will switch to multi-flock. The reason I do not get all worked up about calcium is that from my understanding: the research that talks about excess calcium causing kidney damage is based on research done on meat birds, which are prone to all sorts of systemic issues anyways due to their breeding. How many flocks have roos that eat the same layer as the hens? How often do we hear about sick roos? Most roos are the best looking birds in the flock and live to a ripe old age unless they meet the crock pot or a predator. Look at the diet of a free ranging flock: Many of the greens that they tend to gorge on are very high in calcium. I think white clover is 10%.
Using MF as your main feed source is an excellent way to go. I simply choose not to, b/c it is more expensive. By fermenting my feed, their protein content is high enough with layer, and I don't have the added expense of continually buying oyster shell. When I am feeding MF, I do put out oyster shell. But, that single bag has lasted 4 years.
So, choose your own path. Do the research, consider if the "they said's" make much sense. Realize that no matter how you raise and manage your flock, there will be a chorus of folks to tell you that you are doing it all wrong.