I came across this as I was wondering what the difference between these two numbers were as well. I don't normally add anything before the boil. I always do BIAB, however I end up adding extra water between the mash and boil as my kettle is not big enough to handle the full volume of water plus my grains.
Should I be taking two seperate measurements before the water top up, and after water top up? Either way, would my numbers be the same and if not would the later be my "Meas Pre-Boil Gravity"?
Thanks!