Feels like I got ripped off, but NW reservation agents says it is not so... I still think I got short-changed

Recently, my credit card was billed $74.62 for a series of transactions indicated below.

Total value of original ticket (AZO-TUS) $449.96……(A)
PD887 Coupon $-75.00
Adjusted value/Credit card charged $374.96…... (B)

Total value of the ticket reissued (AZO-PHX) $399.58…… (C)
Price Difference $ 24.62…… (B-C)

Reticketing fee $100.00....(D)

Adjusted value $124.92....(B-C + D)

Minus $50 TCV I supplied -$50.00
============================================
$74.62 charged to CC

Yes, I know it is a long series of transactions, but that is how NW charged me $74.62 after I got the ticket re-issued. Here is my problem.

Northwest Airlines determined the price difference of these two tickets based on the new ticket’s value (C - $399.58) and the adjusted value of the original ticket AFTER PD887 Coupon has been applied (B - $374.96). So, price difference was determined as $24.62 ($399.58 - $374.96 = $24.62). My contention is that the above calculation method is incorrect. Instead, the correct method should compare the straight values of original and the new tickets, i.e. (A) $449.96 and (C) $399.58.


PROPOSED METHOD OF CALCULATION

Total value of original ticket (AZO-TUS) $449.96……(A)
Total value of the ticket reissued (AZO-PHX) $399.58…… (C)
Price Difference $-50.38…… (A-C)

Reticketing fee $100.00....(D)
Adjusted value $49.62 (A-C + D)

Minus $50 TCV I supplied -$50.00
==================================================
$0.38 NWA owes me

The proposed method produces a refund of $0.38. Please note that this is the same amount I would have received from NWA had I NOT used the PD887 coupon in the original ticket purchase. One of NWA reservation agents I talked to vehemently claimed that this method was not correct, I was “not making sense” and “didn’t know what I was talking about”. I have endured these humiliating comments and not being listened to, but was never able to get her to see the simple fact below.

Had I not spent the $75 PD coupon, NWA would have refunded me $0.38, as the above calculation shows. Employing NWA's logic (first calculation), I gave up $75 coupon value to NWA, and I now owe $74.62 to NWA. So, in short, I gave up the $75 PD coupon, PLUS I end up owing more money to NWA than had I not spent the PD coupon?

Intuitively, this is a flawed logic. It is flawed because the current calculation method used by NWA compares reduced ticket value of the original ticket (including coupon value) to a straight ticket value of the new ticket (not including the coupon value). NWA should figure out the price difference by comparing the two straight values, and not mix those apples and oranges.

Maybe I am wrong, but the NWA reservation agent and I could never get on the same page, so I gave up talking to her. In her mind, she must've thought I was just trying to rip off NWA. Hey, at least writing about it made me feel listened to. Sorry if I confused you all.