Here is one that is not on the web but is helpful. Jones rule of 86. Divide 86 by the sugar content to figure out how many gallons of sap it will take to make a gallon of syrup. Example: 2% sugar, 86 divided by 2 is 43. It will take 43 gallons of sap to equal 1 gallon of syrup. It's not perfect but it gets you mighty close.