I am new to Zoho but have used many different systems and struggling to find the correct way to achieve this which should be quite simply, below is what i am trying to achieve.

Fields:

$ - Contract Sum

CheckBox - BG Required

$ - BG Amount

Basically what i am trying to achieve is when the BG REQUIRED Check box is ticked i want to automatically update BG AMOUNT with 5% of CONTRACT SUM

I am struggling to find the right place to get all this working, i have tried function fields, workflow rules, Field Updates, Custom Functions but nothing seems to be able to do all of this or i can find the right place

Thanks