Timing of cross-over point will always be dependent on the season/weather - not the calendar date! Averages are great but reality is what matters!
In a strong, well provisioned, healthy colony in a sensibly insulated hive, nearly all the bees should go through the winter - until brooding starts in earnest. That is when things can easily go wrong (the weather again!)
Most colonies go into winter with ‘around’ ten thousand bees - it matters not how big the physical hive might be. Those bees consume their stores, basically to keep themselves warm, until brooding accelerates. After that the nurse bees will have a finite lifetime.
If, for any reason, brooding is interrupted after starting to accelerate (too little pollen (protein)) for larvae development, insufficient honey stores (possible starvation or isolation starvation), no access to water due to a spell of freezing weather) those colony numbers/imago:brood ratios will change in an unwanted manner.
Then there is the varroa side of things to consider. Those that have a high percentage of winter bees that have been compromised by varroa (particularly at the pupation stage) will lose those winter bees much faster than a good sized, strong, healthy colony. There are other factors, too.
Under-sized colonies, going into winter, fare far worse than strong colonies - for obvious reasons when you think about it. It is why we, as beekeepers, treat nuc-sized colonies with special care (nuc-sized boxes, for a start). ‘Under-stored’ colonies need feeding to get then through the winter. Etc, Etc. Even mini-nucs can be safely over-wintered with careful management.
