Let's say you have a course made up of both Theory 
worth 80% of the student's final grade and Practical
worth 20% of the student's final grade and sub-categories
as follows.

Theory 80% of final grade
  Tests 40%
  Labs  60%

Practical 20% of final grade
  Experiments 50%
  Tests 50%

The best way to approach this is to treat Theory and Practical categories
as if they were a different terms.  Create the Theory term and then choose
TERM - NEW TERM from the file menu to create a second term file to hold the
Practical grades.  Then use the TERM - NEW TERM option one more time to create
a new database file to hold final grades from both the Theory and the Practical
databases.  In this database you can weight the Theory at 80% and the Practical
at 20%.  This database would be analagous to the final average database in
the regular use of terms.  For more information read the following information
on handling multiple terms: 
http://1st-class-software.com/cgi-bin/fom.cgi?file=32