How do I make a built in field required?

[dwp]

I am running version 4.2.0 and am playing with the layout editor. What I want todo is make a built in field required but I don't see how this is possible? I know I can create a custom field and make it required, but I would like to just use the already built in field and add the required bit.

Real world example:
On the Opportunities page there is a built in field called "Percentage" I want to make this field required for each opportunity.

Is this possible? Thanks for your help guys.

[dwp]

Due to the lack of responses I am guessing this is not possible. Any sugar experts willing to confirm this so I can sleep at night?