Lower it until the predicted OG from BM matches the 1.042 you measured.
There's no right answer, it's just one of those things. And it depends on lots of factors, hence it's different for everyone due to their own gear and process.
One you have a number - I'd guess it'll be mid to low 60-ish% - plug that number in next time you plan a recipe in BM.
You should find next time that you end up closer to your target OG. Just bear in mind it might fluctuate a bit at first as you find your feet.
Mine is pretty constant now but still sometimes fluctuates +/- 1 or 2%.
(The one exception was my Baltic porter but in that case I pretty much doubled the amount of grain I would normally use!)
There's no right answer, it's just one of those things. And it depends on lots of factors, hence it's different for everyone due to their own gear and process.
One you have a number - I'd guess it'll be mid to low 60-ish% - plug that number in next time you plan a recipe in BM.
You should find next time that you end up closer to your target OG. Just bear in mind it might fluctuate a bit at first as you find your feet.
Mine is pretty constant now but still sometimes fluctuates +/- 1 or 2%.
(The one exception was my Baltic porter but in that case I pretty much doubled the amount of grain I would normally use!)
