Where customer Credit limit is checking in x++ while Sales order posting

Hi All,

I have done the setup for Customer credit limit and while posting/ confirming sales order its showing error correctly.

Now i need extra validation like there is extra credit limit(custom field) will give addition to actual credit limit for some customers.

While checking credit limit , it has to check that extra credit limit also.

I don't know where to write code for the above.

Please suggest.

Thanks & Regrds,