bonsai said:I was trying to get a handle on how much was 'actually used'
at the end of the dayrather than 'reserved' space.
In that case the answer, as JT has said, is probably not much, if you have enough RAM. I have 512MB and rarely use it all. However, I was recently using an older system with 256MB, since my PSU died on me, and once I had 2 instances of Sierra, 2 or 3 Internet Explorers, Excel and Visual Studio loaded the system became pretty well unusable, it was spending so much time swapping.
So in practice I'd rather make sure I never need to use the virtual memory at all, by having enough of the phyisical stuff!
However, even if you have loads of RAM it's probably not a good idea to make the pagefile too small, you can apparently hang the system if it tries to swap and the pagefile is limited to too small a size.