All you need do is:
1. Give each of the dropdowns a bookmark name (e.g. Theoretical, Manifesto); then
2. Insert a formula field on the 'Overall mark' line code as -
Code:
{IF{=({REF Theoretical}+{REF Manifesto})/2 }= "!*" "" {=({REF Theoretical}+{REF Manifesto})/2 \# 0.0}}
Note: The field brace pairs (i.e. '{ }') for the above example are all created in the document itself, via Ctrl-F9 (Cmd-F9 on a Mac or, if you’re using a laptop, you might need to use Ctrl-Fn-F9); you can't simply type them or copy & paste them from this message. Nor is it practical to add them via any of the standard Word dialogues. The spaces represented in the field constructions are all required.