Just FYI, if you ended up centering both flanges between drops, you'd do better not to use an offset rim. The reason for an offset rim is to make spoke length on the left and right side more equal by compensating for the unevenness of the flange placement on the rear hub. The rim is built with the spoke bed off center, so the drive side flange is a further reach, and the non drive flange is a shorter reach for the spokes.

O/C rims are the great equalizer for the unevenness that goes on with rear wheels. Spoke tension on the non drive side is much closer to drive side tension on an o/c wheel than on a wheel with a centered rim. All new multi-speed rear wheels should use them.

But if you centered the hub flanges between the drops (not that you should), using an o/c rim would require you to build a wheel with some dish even though you could and should build it dishless.

As for spoke length, rear wheels typically have a 1-2mm difference in drive to non-drive side. With centered rims, left and right are most commonly 2mm apart in spoke length. With o/c rims, a 1mm difference between ds and nds lengths is most common, with the left still slightly longer. Calculations vary among hubs and rims. A 4mm offset is pretty typical with o/c rims. If I had to guess, I'd probably enter a three and call it a day. Other folks are right that certain dimensions don't change your spoke length all that much. ERD and flange diameter are by far the biggest determinants.

Finally, with a 1mm difference in spoke calc results you can use one length no problem. Some spoke manufacturers sell only in 2mm increments. In your case, I'd probably use 292mm on both sides.