This is an old family recipe.  In recent years, I have changed to using sure-jell, but this is a very good rule.

2 lemons
1 dozen oranges

Grind fruit and add 3 pints water to each pint of fruit.  Let set overnight.  Cook 2 hours.  Let set over night again.  Cook 1 pint sugar to 1 pint juice.  Boil to jelly stage.  This will take about 14 lbs. of sugar and yield about 14 pints marmalade.  It’s easier to cook 3 cups liquid each time.