match first occurrence for which a condition holds tru

Hi all,

i'm struggling a bit with following simplified use case. I have a subpart
which consists of 1 or more materialgroups and a materialgroup consists of 1
or more materials.  They all have a percentage (measured on subpart level).
Sometimes the sum of these percentages doesn't add up to 100% so the customer
wants me to correct the largest [first matching] percentage so that the total
percentages add up to 100.

I am able to match on the percentages which match the maximum in the context
of subpart but in example below i match on both lead and trioxide whereas i
only need to fix the first occurence which is lead in this case.

How can i solve this elegantly?

Kind regards,
Robby Pelssers

  <xsl:template
match="MassPercent[max(ancestor::subpart/materialgroup/material/percentage) =
.]" mode="fixpercentages">
    <percentage>fix me</percentage>
  </xsl:template>

<subpart>
  <materialgroup>
    <material name="lead">
      <percentage>35.30</percentage>
    </material>
  </materialgroup>
  <materialgroup>
    <material name="trioxide">
      <percentage>35.30</percentage>
    </material>
    <material name="Polybutylene">
      <percentage>20.50</percentage>
    </material>
  </materialgroup>
  <materialgroup>
    <material name="Glass Fibrous">
      <percentage>08.80</percentage>
    </material>
  </materialgroup>
</subpart>