Send first invoice reminder email by days before the due date

I would like to have my first invoice reminder email sent 7 days before the invoice due date, regardless of what the payment terms are. However, the first reminder email can only be set for a certain number of days after either the invoice date or due date. Not all clients have the same payment terms. So if the first reminder email is sent 23 days after the invoice date, that is 7 days before the due date for clients with 30 day payment terms, but 22 days before the due date for clients with 45 day payment terms.

Currently it is possible for the second and third reminder emails to be sent a certain number of days before or after either the invoice date or due date. It would be helpful if this feature were added also to the first reminder email, so that I could set it to send 7 days before the due date.