The manual says to fill it to the top, but that will be technically a little overfilled. The idea is that it will vent out of the side until it finds its own level and should be pretty happy from there. I can't for the life of me work out why the Ferret was never fitted with an overflow bottle - Photographic evidence shows the Brits mounting them on trucks during WW2, so they certainly knew about them...
That being said, when I first got my Ferret, the vent valve was faulty and was not building up the required pressure in the system, resulting in it constantly venting and steaming/boiling off. They are a two way valve and should be closed under normal conditions and should vent outward at a fairly high pressure and suck back in at a lower negative pressure (they are actually two valves, one inside the other). I replaced mine with another one from Empire Trading and it's been good ever since. The supplied valve was slightly different to the one on the vehicle though, and I had to drill out the mounting holes a little to have it fit properly.
Cheers,
Terry