specific discount

Hi everyone,

I'm trying to implement a specific discount per account in the CRM. So, when i'll create, for example an invoice, the amount of the discount will automaticaly be added to the grand total.

First, i've created a custom field (in %) for the accounts called "specific discount (%)".
Then I wanted to create a custom formula field in the invoice (and quote and order) with this formule:

${Invoices.Sub Total}-${Invoices.Sub Total}*${Accounts.Remise spécifique (%)}

But I think we can't add a field which is not in the ${Invoices....}.

Does someone know a way to go around this problem?

I think the easiest way will be to create this field in the account section, and then do something with will automatically import this data in the invoice section. But i don't know if it's possible or not.

Anyway, I hope you'll be able to help me guys!

EDIT:

Also, does someone know how to show the discount rate (of the normal discount, not the specific ;) ) on the invoice/quote/order?