There are others more familiar with cross tab reports than I am. Having said that you may want to consider doing the divison in the SQL query as an SQL expression.
You could add a line something like this to your query,
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(quantityfield/numberfield) AS Percentage,
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What I cant tell you is how to add the expression field to your crosstab report since it is summarizing the data. Perhaps you could consider a different style of report.
I am sure others on the forum can help. Maybe this will create more ideas from them.
Hope this helps, let us know if you find a solution so we can all learn.
Regards,
John W.