As you can see the percentages in the xml snippet before transformation add up to 99.90%.
The customer wanted to make a correction on the first maximum percentage so that all percentages would add up to 100%.
So this means adding 0.10 to the "lead" material percentage. A little inquiry on the xslt mailinglist showed me how to solve this elegantly.
Special thanks go to David Carlisle for the feedback.
BEFORE:
AFTER: