You can either calculate it based on a domed cylinder, or do it empirically by measuring our liquid.
If you decide for the latter (which I like because it includes pipe volume) do something like use a 100ml measuring cylinder to measure out 1l accurately into, say, a soda bottle, mark the level on that, then 10 of those into a bucket for 10l, mark where on the bucket it was, then you have nice 10l increments to keep adding to the processor.
I shall mark my new processor in 25l steps, otherwise there will be too many
