Strange total when I exclude tax from discount.

Could this be a round error in the calc somewhere ?

This is the order when the discount is calculated excluding tax

Order total inc tax 18.50
tax is 19%

So 18.50 / 119 * 100 = 15.546
So discoint before tax is 10% of 1.55
discoint shown 1.55. correct

Now when i do the same but with tax included in the discount.
10% x 18.50
Discount shown = 1.86 instead of 1.85

Zen Cart 1.3.7
Database Patch Level: 1.3.7

I also have giftwrapping installed.

Discount Coupon settings

This module is installed
true

Sort Order
280

Include Shipping
false

Include Tax
true

Re-calculate Tax
Standard

Tax Class
--none--

Looks like you are on the right track in the order totals page, just looks like you may have to alter the sort order so that the discount happens after sales tax calculation?

Then again we have discount coupons in fixed amounts only - and have it loaded as second to last activity to happen.