To produce a proper port, sherry or madeira, you will need to add distilled spirits to your wine.  This process is called "fortifying" the wine.  The PEARSON SQUARE is used to determine the amount of alcohol needed to fortify a wine.  It is necessary to know the percentage of alcohol by volume of the wine to be fortified, also the percentage of alcohol by volume of the spirit to be used for fortification.  The spirits most often used for this purpose are brandy and vodka of 40% alcohol by volume (80 proof).  Please note that if you know only the proof of the spirit, then you must convert this to alcohol by volume.  This is easily done by simply dividing the number proof by 2.  For example 80 proof is 40% alcohol by volume (80 ÷ 2 = 40).
  • A = Alcohol content of spirit to be used.
  • B = Alcohol content of wine to be fortified.
  • C = Desired alcohol content of final product.
  • D = C - B This is parts of fortifying spirit.
  • E = A - C This is parts of wine to be fortified.

Let's say you have five gallons of wine at 15% alcohol that you would like to bring up to a total of 20%.  You are planning on using brandy at 40% (80 proof) to fortify this wine.   Therefore,
  • A = 40
  • B = 15
  • C = 20
  • D = 20 - 15 = 5 parts fortifying spirit
  • E = 40 - 20 = 20 parts of wine
Therefore for every 20 ounces of wine that you need to fortify, you will need 5 ounces of brandy to bring the final product up to 20%.  This is a 4:1 ratio of wine to spirits (20 ÷ 5 = 4).  Each gallon of wine has 128 ounces.  This means you have:  5 x 128 = 640 ounces of wine.  Divide this number by 4 and this will tell you how much brandy you will need for this five gallon recipe. 
640 ÷ 4 = 160 oz.
Yes, that's right. . .160 oz. of brandy!  That's over a gallon!  As you can see, it is a good idea to derive as much of the alcohol from fermentation as possible.  Sugar is much cheaper than brandy!  You may also want to consider using Everclear to fortify your wine.  At 190 proof it will require much less of this spirit to reach the desired alcohol content, plus you will dilute the flavor of the wine less because you are using less than half as much distilled spirits.