I just finished brewing my first beer. When I was transferring the wort to the Big Mouth fermentation jug, the directions said to top it off with water up to the 1 gallon line I previously marked. However, it was maybe only 2/3 of a gallon of wort. Is it fine to top off with that much water?
yep, adding 1/3rd gallon of water is just fine.
With extract brewing your OG is already set to a specific volume as long as you use all the extract. Therefore, if you got a little more boil off than expected and you used all your extract, you can add enough water to hit the specified volume and should have the same OG.
Should be fine. Once you feel you got your brew system under control. And you know what your boil off rate will be start trying full boil. So you dont need to add water later. You might want to add than half of your extract. Bit later like 20 min before end of boil. So your brew dont become that dark. Think they call it mailard effect