If you have a beer that is 1.040 SG, and you have 5 gallons, you can do the following
1.040 = 40 pts/gal x 5 gallons = 200 pts
If you add 0.5 gal of watermelon juice at 1.020, you will be adding
1.020 = 20 pts/gal x 0.5 gal = 10 pts
You now have 210 pts in 5.5 gallons, or
210 pts / 5.5 gal = 38.18 pts/gal = 1.038
