You just take your SG testing tube and fill it enough to get an SG reading. That's a highly concentrated product and since it's a little past it's "Best By" date you might want to shake it a bit before trying it. Keep in mind if you aren't able to get a reading because of the concentration, then take your testing tube and fill it about 3/4 to 7/8 full. Measure how much water that takes. The recommended dilution rate is 1 part wine base to 4 parts water (IF you are making 5 gallons from their 1 gallon jug) So figure out that dilution rate. So if it take 5 oz for water to get to the 7/8's mark on you testing tube use 1 oz wine base and 4 oz water stir put that in your testing tube. That won't diminish your full jug volume that much and you can then figure our what you might need to add to get the desired ABV. That diluted testing sample should also be pH test as well.
That's not that hard now is it?
I'm not a fan of that brand of wine base but some folks love it. So since you have it - don't let it go to waste.
As for the idea of mixing real blueberries with this wine base. My normal blueberry wine, from my home grown blueberries, is made with 6 lbs of blueberries per gallon. It takes about 8 lbs to get a full gallon (before fermentation) but personally I seem to start to see problems with too much acid at that rate of berries per gallon.
So the rest of the recipe is just a matter of following a basic fruit wine recipe.
Some additive amounts are based on the state of the must ((
A)Sugar, and (
B)Acid Blend)
(
C) Tannin for Blueberries isn't necessary but won't hurt anything if you like the mouth feel of tannins. The remaining additives don't really vary from one fruit to another.
(
D) Yeast Nutrient (Follow Nutrient bottle directions)
(
E) Pectic Enzyme (Follow directions on the bottle) {Some fruit varieties are need a LOT of this to break down the fruit and clear it but blueberries are pretty good. with just a normal dosage.}
(
F) K-Meta would not be needed except if you are using fresh blueberries then you want to kill any stray bacteria and stun any wild yeast cells.
If you have any more questions ask away. As I said a number of folks like Vintner's Best. I personally just prefer my fruit wines to be single variety unless I am trying something different.
Remember in the future you can find all sorts of recipes here on this forum :
Recipes
For questions about non-grape wines this is an excellent section of this forum to look for help:
Country Fruit Winemaking