If you have a really accurate scale, weigh out 1/20th of a pound (.08 ounces) of molassas and mix it into 1/20th a gallon (6.4 fluid ounces) of water. Use your refractometer to test the gravity. This would be the same as a pound of molassas in a gallon of water or PPG. You can scale down the proportions as much as you want but it will require a scale with really low resolution.