Hi Dave,
Thanks for the comments.
With white backgrounds - if you light them seperately you only need to go about 0.5 to 1 stop above what's required to light the foreground (after all, the background is already white) - UNLESS ...
... the backdrop is a sheet with creases in it - in which case you have to "nuke" it about 2 stops over. Unfortunately, I'm stuck with a white "sheet" until my white seamless paper arrives (ordered last week - chased today - no sign of paper nor reply from account rep )
All good things that the testing phase is identifying