Would it be possible to improve this stylesheet (or to make a new one)
  so that it didn't require pre-calculating?


i think that pre-calculating the coordinates is the right thing to do
you don't need to do it by hand though: you can do it with xslt, either in
a first pass stylesheet or in the same one, then use exslt:node-set to
run your existing stylesheet on the modified input.

David

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