Survey:Add formula to survey field

Hi ,

I have the requiement that I need to add a  field in the survey (CRM_SURVEY_SUITE)  , that should be calculated based on the values entered by users.

Input field - A - 80 (user enters)

Input filed - B - 100 (user enter)

Input field - C should be filled with 80%(calculated using formula).

This calculation shuould be calculated on survey save(Function code SUBMIT) button.

I checked PAI and PBO function modules , I can't find where to enhance and keep my formula.

Please help ..

Thanks,

Naveen.