Go up into the attic and add blocking betwen the joists. Make sure the blocking is firmly attached to the joists, and use the big lag screws to attach the hooks for the chains for the rack. If you don't have attic access, you'll have to cut out drywall from below and then patch and redo.

Thanks so much for your response. We do not have attic access there as there is a second floor over that part of the house. We were hoping to be able to avoid cutting out drywall, ( I guess by 'any good way' I meant 'any easy way' LOL) but had suspicioned that might be the only way to center it - too bad for me! I guess my decision now is no pot rack, or more of a project than we'd hoped for. Thanks again.

Nothing is ever as easy as it looks. :-)

You could attach solid wood on the underside of the drywall (where you can see it)
I would think there is a way to somehow center THAT and attach the rack to it.
Is it possible for you to post a pic somewhere with dotted lines representing the floor joists above your ceiling?

We did as craigbass is suggesting for our wall mounted potracks. We painted the wood the same colour as the wall and it is barely noticeable.

Thanks again for all of the suggestions. Thought you might want to see what we ended up doing. We really wanted to avoid cutting into the ceiling and having to patch it, and wanted to have it up sooner than it would take (at least for us) to fashion something out of wood that would look nice. So we ended up coming up with a work-around (AKA we kind of rigged it :-)). I'm crossing my fingers that the pictures I'm trying to post will come up here. The anchor points in the ceiling are not centered, so we hung bars from those points, and were then able to center the rack from there. Here are a couple of pics with no pots:

..and a couple with the pots:

I know it's not perfect, but I don't think the fact that the anchor points are off center is TOO noticeable, and if we decide we hate it or something, we've only got 4 small holes to patch in the ceiling. Let me know what you think.