I'd forgotten about the slider. I've now increased it from 10 to 12% but on re-opening, although the 12% has stuck, the value of 3.99 GB hasn't changed. Maybe needs a reboot? BTW, 10% of my 750 GB HD, or around 690 GB actual, is a lot more than 4 GB, which is puzzling. Is it perhaps 10% of whatever space is left?
Those rules now make sense of the behaviour I'm seeing.
Terry, East Grinstead, UK