XPath - How to select preceding nodes that are descendants o

I've got a problem that I can only seem to solve using node-set - and I
can't use node-set in my current XSLT processor.

I have an XML document with multiple subtrees starting at element
'SUBTREE' containing 'LEAF' leaf element descendants at various
hierarchical depths.
I need to know - for each 'LEAF' element - how many preceding 'LEAF'
elements there are within the closest ancestor 'SUBTREE'.

The document has many SUBTREES - so if I just use the 'preceding::' axis
and do a subtraction the processing time is horrendously long.