no point at all using a relative path the behaviour is going to be mostly unpredicatable. the better approach is to define the save path in either a property file or in the web.xml file (deployment descriptor). Eitehr way you can then customize it accoding to the installation.
How to pass in additional parameters to servlets (or jsp as the case may be) is well described in the tomcat documentation - there is even a sample you can use.
for using property files you can read up the tutorial on the java.sun.com site or the API doc for the java.util.Properties class - it's really simple.