As per this recipe for a Belgian Dark Strong Ale, I'm brewing a beer where it's suggested you take a portion of the pre-boil wort, and boil it down separately to create a syrup to add back to the main wort later.
Given this, how should I calculate my pre-boil volume?
a) Use the same pre-boil volume as usual, but take a portion of this to boil into a syrup
b) Create a larger than usual pre-boil volume, to cater for the amount I take away
c) Just add some boiled top-up water at the end to compensate
d) Something else
I all-grain BIAB, and would need 9.75L (90 min) pre-boil wort for a desired post-boil vol of 6L