I'm building a website that allows users to search a FileMaker Pro database, but I'm having trouble creating "Back" and "Next" links using CDML.

I can't use the regular CDML tags (<FMP-LINKPREVIOUS> and <FMP-LINKNEXT>) because I am using a <base> tag to set the location for all images and links, and the <FMP-LINKNEXT> etc use relative links and thus don't work.

Anyway, I've worked out how to do the "Next" link:

<a href="http://mywebsite.com/FMPro?-db=contacts.fp5&-format=searchresults.htm&-lay=web&-sortfield=surname&-max=10&-skip=[FMP-rangeend]">Next</a>

But how do I do the "Back" link in a similar way?
I want to be able to subtract 11 from the rangestart variable. How do i do this?

<a href="http://mywebsite.com/FMPro?-db=contacts.fp5&-format=searchresults.htm&-lay=web&-sortfield=surname&-max=10&-skip=[FMP-rangestart-11]">Back</a>

Any help would be much appreciated.