This is probably a dumb question but ill ask anyway , if you take a preboil gravity rating it will be exactly proportional to your post boil gravity , correct? Ie you won't boil off any sugar.
Correct on the sugar not boiling off. Though the gravity and volume wiill be inversely proportional...i.e. the product of multiplying those two values will be constant.
You'll also have to worry about losses to hops/trub. Just because you start with 6 gallons of 1.050 wort, doesn't mean you'll end the boil with 5 gallons of 1.060 wort. More likely you'll have 4.5 gallons of 1.060 wort in the fermentor with half a gallon lost to other places.
The Abyss Yeah, 1 gal with 100 points of sugar total will make 2 gal of wort with 50 points of sugar total. If you boil off a gallon from that, you'll be back to 1 gal with 100 points of sugar total.
