Other questions

RE: $10.30 * 0.60 comes out to be exactly $6.18

This suggests the discount is 40%, not 60 cents.

I set up a Price Level with a 40% discount and it worked right for me. For an item that cost 10.30 each, the resulting price for the price level is 6.18.

How is your price level set up? Does it look like this when you edit it? Something different?

RE: I tired changing the price to$10.28. Manual calculation, $10.28 * 0.60 = $6.174.  QuickBook has it as $6.18. 

10.28 * 0.6 is 6.168, not 6.174.  When I tried it in QB, I got 6.168 as the rate, as expected.

It seems like QuickBooks is consistently both rounding and adding a cent in your case.  That could be a product of the rounding setting on the price level, if you've selecting anything but "no rounding" as seen above.

I get your results when I set the price level up like this: