If it doesn't specify on the bottle, I'd try 150ml in 15 litres and see how it goes.  15 litres is a bit heavy (especially this time of year).  I'd go for 100ml in 10 litres.  You can get measuring cylinders at the Farmacia.  I measure out 100ml into the cylinder and then add the contents of two spare 5 litre water bottles.  Try and lift the sprayer onto a raised platform of some sort so you can back onto it.  Difficult to get it on your back from the ground.  Also make sure the filter in the spray head is clean before you start.  Sorry if I'm teaching you to suck eggs.
Bigyin,
P.S.  10 litres should be enough for roughly 1,000 square metres.