Page master selection. .fo, db5

I note that for my usage, I can see 15 page-masters.
Each has the same paper size.
Each has the same margin-top margin-bottom margin-left margin-right 
(inside/outside inverted)
Each has the same region-body margin-bottom, margin-top, margin-left, 
margin-right.
Each has the same before-extent, after-extent.

How would I (for instance) change the dimension C in the diagram at
http://docbook.sourceforge.net/release/xsl/current/doc/fo/general.html
Perhaps to set the opening text in a new chapter a little lower?

Is it possible to configure these via params? Another way?
Or should it be done using (say) the space-before in the title?


TIA

regards

-- 
Dave Pawson
XSLT XSL-FO FAQ.
http://www.dpawson.co.uk