The "calculation" isn't required at all in setting your sell price. It is only there so you know your minimum.
You want XXXXX credits for an item, put it up for that. If it sells, either to a buyer or a reseller, put it up higher.
If it sells again, put it up higher. Eventually, it won't sell, as you have reached the price limit (Which I would suspect is closer to your 12k, maybe even higher if they say worked @15k)
Once you've found the price limit, it's time for the AH PvP to begin with other sellers. You know the point you can't afford to sell at...lets say 2k...and they are listing from 5k-12k. Now, the market you say is allowing you to sell a few, and reseller seems to be getting a few sales as well, so competing will be fun for all