Invoice entry AP20.2 + Discounts AP22.3

New user here (3rd day) and I have a couple of questions which I've asked my staff and they did not know.

(1) How do you enter discounts within the AP20.2 {TAXED INVOICE} module on the Main tab? I do see a "Discount" box and have clicked on that, which took me to AP22.3 {DISCOUNT}. My staff instructed me to enter the discounted amount of the invoice in the INV AMOUNT box and then use distribute out the cost to the various GL account(s). The PMT TERMS box is indicated with "N00" = pay immediately.

What should be indicated in the PMT TERMS box? We do not have 29091 terms. The closest we have is 26090. How does this affect the DUE DATE box? If nothing is entered in the PMT TERMS box, but a date is entered in the DUE DATE box, what is the effect on when the invoice is paid / check is generated?

INVOICE DATE = 10/09/2014
TERMS = 2% 90, NET 91
PROMPT PAY DISCOUNT = $175.36
AMOUNT DUE IF REC'D BY 01/07/2015 = $9,267.64
AMOUNT DUE ON 01/08/2015 = $9,443.00

(2) If the DISCOUNT box is clicked, which takes me to AP22.3 {DISCOUNT}, what should be entered in the boxes in this module (e.g. Discount Rate = 2 or .02 for 2% discount)? What should be entered in the AP20.2 {TAXED INVOICE} to ensure it is accurate?

I am looking at setting up these invoices to pay / checks to print approximately two (2) weeks in advance of the due date so we can process, print and mail the check in time to receive the discount.

(3) Previous invoices I have entered the discounted amount in the INV AMOUNT and the GL DISTRIBUTION boxes. No PMT TERMS were indicated, but a future date was indicated in the DUE DATE box (e.g. discounted amount due = 12/10/14 and the date in the DUE DATE box = 11/24/14). How is the invoice / printing of checks affected by this?