Best way to hit target OG is to work in "gravity points". It's sounds a bit complicated at first but is extremely easy to work with, especially for all grain brewers. Here's the way it works:
If target = 5 gallons of 1.050 wort
-eliminate decimal places an multiply times gallons (50x5) to get 250 points.
-now just calculate as you go; let's say your first 2 gallons show 1.070
-that's 70x2 or 140 points.
-let's say the next 2 gallons dilute it down to 1.050
-that's 50x4 or 200 points
Let's say you know your boil off will be one gallon, so you want to collect 6 gallons. What gravity will 6 gallons have to be to hit 1.050 at 5 gallons?
-simple! 250 points DIVIDED by 6 gallons, (41.66) or 1.042
If you're under you can either collect more and boil off more, or add malt extract or other fermentable. If you're over you can add water and make a bigger batch, stop the sparge and add water, or make stronger beer. Either way, you're in control, not the author of the recipe...
PS, you can see the value of a refractometer here; many gravity samples are required...
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