I had used 100gr DME/1000ml H2O using a 15min boil.. When i finally measured, 15min boil = 750ml @ OG 1.055.
I never accounted for the boil off vol at 15min nor the resultng higher gravity. DOH!!!!
I now boil 1250ml H20 to to get 1000ml after a 15min boil. I also adjusted my DME to 90gr /1000ml and now get an OG ~1.035. YMMV
Are 15min boils necessary sanitze the starter wort?