I am currently reading John Palmer's How to Brew and in the book he has a chapter with a few of his favorite recipes. On each recipe he has the following, "Boil gravity for x gallons" (x being the number of gallons). For example on recipe "Confident Bastard - American Strong Ale" Version : Extract and Steeping Grain" he has listed under Wort A, "Boil gravity of 3 gallons" 1.059. What does boil gravity of 3 gallons 1.059 mean?
That means the three gallons of wort you are starting your boil with has a specific gravity of 1.059. The wort is more dense than water (specific gravity 1.000), the specific gravity is a measure of how much sugar is in the wort (dissolved solids, actually, because all of them contribute to the density of the wort).
Both or either will suffice. I've not got a refractometer but less wort is needed to measure gravity and it's pre fermentation so no alcohol in suspension to scue the reading.
