I was wondering how do you normally decide how much water to add to the blueberries to give you an exact amount of liquid.

Example, if I plan on 4 gals of blueberry wine, 3 gal for carboy with an extra gal to use for top ups. I wanted to use 7lbs per gal of blueberries. If I crush 28 lbs of thawed blueberries, and place in fermentation bucket, not all the juices have been extracted. How do I know how much water to add to give me that 4 gals? Does that eventually just come with trial and error and taking good notes or is there a technical way to calculate it?

Seems if I just fill to 4 gals, When I pull my mesh bag with skins the volume would drop quite a bit. If I pull the berries out before to add water, I may add to much. So just wondering how the proper way to determine the addition of water.

Also would using a refractor be the correct way to measure the brix when you have a bunch of berries to determine how much sugar to add. Since the berry sugar has really not all been extracted, I am thinking I would get a few berries and crush them up and measure the brix from the berry and somehow calculate how much water I'm adding and then what I want it to be and come up with amount of sugar to add? Am I off base?

First, we use a rule of thumb for figuring out how much more volume to make to end up with the final wine volume we desire. It's 65% - 70% of the original must volume. This assumes that you'll lose 25% - 30% of your volume due to racking and pressing. It works on grapes, but is a little heavy handed when making wine that has lots of added water. If you wanted to make a straight blueberry wine, little to no water added, it would take in the neighborhood of 60# to get 3 gallons.

To answer your question, if I were shooting for 3 gallons finished wine and wanted to have 7# of blueberries per gallon, this is what I'd do:

Mush up 28# of blueberries in a bucket and measure the volume of the mush, which I'm guessing will be somewhere around a gallon. If 65% of that mush ends up as wine, you'd get .65 gallons from it. I'd then add 3.35 gallons of water, for a final must volume of 4 gallons. Check and adjust your pH to be 3.3+, blueberries are notable for high acid / low pH, getting it up to 3.3+ will help your yeast out tremendously. Adjust the BRIX up to your desired level so you get the ABV% you're looking for. Add some pectic enzymes, mix well and let sit 12 hours before adding your yeast.

The refractometer is a good way to check your BRIX prior to fermentation, and just know that it'll probably rise a tad once the enzymes do their thing. After fermentation starts, switch over to your hydrometer for BRIX / SG readings, as the refractometer readings are skewed by the alcohol in the wine. There are formulas to adjust the readings, but I prefer to just switch to the hydrometer.