If you want to calculate plato before you add the extract to the water, then you need to know how much gravity your extract will contribute. For instance, say your extract contributes 40 points per pound per gallon. This means that 1 lb dissolved in 1 gallon of water will yeild a gravity of 1.040. Once you know what your gravity will be degrees plato is easy. Lets say your gravity is actually 1.0

40. Degrees plato =

40/4 = 10. If the gravity is 1.0

55, degrees plato =

55/4 = 13.75. In your case, you'll have to convert liters to gallons and kilograms to pounds. I use a handy conversion calculator for such things.

http://www.onlineconversion.com/