[Xymon] obtaining maximum value to calculate percentage for graph

I am monitoring a value for which I don't know the possible maximum, which I would like to express as a percentage.

Expected to be able to do something like this (MAX should be a valid CF):

    DEF:packets=fsth.rrd:freepackets:AVERAGE
    DEF:packetsMAX=fsth.rrd:freepackets:MAX
    CDEF:packetsPercent=packets,packetsMAX,/,100,*

Alas the second line is not valid.

What is the appropriate syntax for obtaining a maximum value?

-- -- David Boldt <dboldt at usgs.gov>

"He who asks is a fool for five minutes, but he who does not ask remains a fool forever." --Chinese Proverb