"David Hill" wrote in message
...
".......... I have an old and sprawling rosemary which needs a serious
chop.
How much can I take off without killing it?........"

I would put in some cuttings, some don't survive the hard cutting that
they
get.

I have grown rosemary all my lengthy adult life and have never lost one
except by intent. Whenever I have had to reduce them, I have simply
followed the rule of making sure that there is at least a vestige of green
growth left below the cut.

Franz