Decoctions are always assumed to be at boiling temps.
BS is a pure numbers tool and does not seem to account for the large amount of lost heat to the actual process of pulling the decoction. I'm guessing it's modeled on an idealized calorimeter but is harder thing to mathematically model in reality.
Offset the lost thermal energy on two fronts.
1. Main mash can be heated directly to maintain the rest temp from which the decoction is pulled. (I use a very low flame on my MT with the reflectix). No fires yet.
2. As Q mentioned, up the volume of the pulled decoction over what BS suggests. That's something that boils down to experience but typically I'm at about suggested volume +25%. Better to have too much than not enough
It's really not too hard to do. I get enjoyment out of doing these silly mashes
Two things I think I think are worth mentioning
1. The effects of specialty malts added for color seem to be magnified so tailor your recipe acordingly
2. Mash pH seems to clock in a bit lower probably as a result of same. (Very small data set so I may be way off base there)
There is a thread in my sig covering some of what I've learned about step mashes. I might be of use. I am far from experienced at the whole thing but have done my best not to disseminate any misinformation therein.
Here is the thread if your on mobile.
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