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I set-up a 10% discount code. The product price, before discount, is $194.75. 10% off should yield a $19.48 discount. But it's only calculating $18.45.

This is an "all items" percentage discount. FYI, it's a quantity of 205, per-item price of $.95.

The reason for the discrepancy is explained on this help page:
http://www.e-junkie.com/ej/trouble.discounts.discrepancy.htm

Normally, that would result in a discrepancy of maybe a few cents, but the same rounding imprecision applied across 205 items is magnifying the discrepancy in your case. A 10% discount amounting to $19.475 off the order total, spread across 205 items, works out to a discount of $0.095 off the price of each item, but we can't pass individual item prices to checkout with sub-cent precision, so that gets rounded to $0.09 off per item, times 205 items = $18.45 total discount.

If this isn't reflecting an actual or likely order, and you're just testing the discount function, try making a dummy product with whatever price you wish, that you can Add to Cart to test with the discount code.