Less than full carboy

I've made a couple of batches and although I seem to have a full kettle and draw off 20 litres, I only have 15 litres when I rack off the true into the secondary, a 19 litre carboy.  I ended up topping it up with water do it is full.  What should I do to make sure I have enough after primary is completed?

Rick

Try increasing batch size by the amount of brew you are losing. For example set out to make a  6 gallon batch to achieve a 5 gallon yield because you will lose about a gallon in total from transfers from the boil kettle, fermenter, bottling bucket, cooling loss- shrinkage, evaporation,etc...
Each time you perform a step that involves a loss, pour your lost brew into a graduated container and see exactly what your actual losses are. Then you can enter these loss amounts into beersmith and it will tell you final bottling volume. Increase the batch size until your final bottling volume is what you want to yield (e.g.-5 gallons).  You will eventually bottle exactly what you expect to get.  CHEERS!