Ah.B  Ok, so if I understand correctly, I need to change the variable
assignment to grab the position of the <DATA> element within the parent
<ROW>, rather than within the for-each?B  How would I do that?

I think what David tries to say is that you want e.g.
  ../../fmp:COL[3]/fmp:DATA[$parent-position + 1]
instead of
  ../../fmp:COL[3]/fmp:DATA[$parent-position]

Or you would need to use
  ../../fmp:COL[3]/fmp:DATA[position() >  1][$parent-position]
  ../../fmp:COL[4]/fmp:DATA[position() >  1][$parent-position]
  ../../fmp:COL[5]/fmp:DATA[position() >  1][$parent-position]
and so on to work in the same range your for-each works with.

there are 4 DATA children so

<xsl:for-each select="fmp:COL[3]/fmp:DATA[position() > 1]">

selects the 2nd, 3rd andB 4th

the scope of that position() is just that step of the xpath, within the
body of the for-each, position() counts the selected items

so as there are three items selected position() as stored in
$parent-positionB  will have values 1,2,3 which is one less than you
expected.