I was recently asked a question about Bhismapancaka that might be of interest to some of the Tattvavivekis, I'm guessing at least Nandatanuja.
The question was something like this. Bhismapancaka is supposed to comprise the last five days of Kartika, beginning with Ekadasi, but if you look at the calendar, that will only make four days, even counting the day of breaking the fast (12.11-15.11 here in Finland). How come?
Answer: HBV 16.434 seems to say "five days beginning with Ekadasi," but that makes little sense, since how could one then break the Ekadasi fast? Also, since Caturmasya begins and ends with Ekadasis in the HBV, if the Bhismapancaka is supposed to end the Caturmasya, it would have to end with Ekadasi, not begin with it.
This is corraborated by the Dig-darsini commentary to the HBV (16.436), which quotes from the Padma Purana that during the four days beginning with Saptami during the bright fortnight of Kartika, one should drink water, milk, yoghurt and ghee, and then fast (totally) during the Ekadasi. That comprises Bhismapancaka. In Finland, Bhismapancaka would then begin on the 8th of November and run up to the 12th, and be broken (with the Ekadasi-vrata) in the morning of the 13th.
In other words, the problem re. number of days has to do with when one starts the Caturmasya vrata. If one wants to start at purnima (and that alternative is given in the HBV as well), one will not be able to do the Bhismapancaka (which is an optional vrata, according to HBV) because 1) there will be four, not five days, and 2) one would not be able to properly break the Ekadasi fast.
1. If I've started Caturmasya vrata at purnima what would be schedule for bhisma-pancaka for our location?
2. Does the end of bhisma-pancaka means end of Caturmasya vrata? Kartika ends on November 15th, but in your case bhisma-pancaka ends on November 12th, so what will be happenning in between?
3. What do you mean by properly breaking the Ekadasi fast? You can take milk during bhisma-pancaka, so if you do Nirjal Ekadasi why can't you break fast by taking milk, so both rules are satisfied?
I still think that it is impossible to do Bhismapancaka if one started the Caturmasya vow at purnima, since the Bhismapancaka is supposed to be the five last days of the vrata (pancaka means "fiver"). According to the Padma Purana, this vow means to take only water, milk, yoghurt, ghee and nothing during five days. If Bhismapancaka should end on the last day of a Caturmasya vrata begun at purnima, it would be as follows:
10.11 Water
11.11 Milk
12.11 Mahadvadasi (Utthana Ekadasi) - Yoghurt
13.11 Ghee
14.11 Full fast
15.11 Purnima - breaking the fast
The fast for Utthana Ekadasi, the most important Ekadasi of the year is on the 12th, the day for yoghurt. This Ekadasi is one of three during which one must fast completely according to the HBV, so one could not take yoghurt. Even if one didn't follow that rule (and most Gaudiyas don't), one couldn't break a "partial" fast by taking just ghee the next day.
Of course, if the rules you follow for Bhismapancaka, Nandatanujaji, differ from those mentioned in the HBV, that may change things. What source are you using?
I just noticed that the calendar for Audarya differs from that for Finland in that Utthana Ekadasi is celebrated on the 11th instead of the 12th, but that doesn't help.