I don't really think that buying a fix amount of Bitcoin when talking about DCA is proper because to my own understanding, if you really want to buy through the DCA method, then you have to buy with a fix amount of money, weekly or monthly irrespective of the current price of Bitcoin, but if you want to buy a fix amount of Bitcoin, them you might spend more or less depending on the current price of Bitcoin, but the major challenge with this strategy is that you will be forced to tamper with you emergency fund that would have been for your upkeep till month ending when you are expecting money from another source.
I just believe that the DCA method is design for us to buy in a comfortable manner, not in away that will make us to start doing opportunity cost just to meet up to the target of buying a fix amount of Bitcoin.
Basically as an investor, it is believed that you understand your capital flow and how to strategically place your investment. Before you would want to buy Bitcoin using DCA Method, it believe that you have a capital base you use to sustain your buying, wether small or big amount for the period you want to. Don't also forget that you're buying with a certain percent of your income with the hope of Holding for a very long time.
Emergency funds doesn't really come into play except for a situation whereby you don't have the financial base to continue with your investment pattern, that is why is always advice for you to know the period of time it will take for you to strategize your investment pattern based on the funds you have, so you wouldn't find yourself in an emergency situation, so opportunity cost don't have any role to play.