I enter the exact width of the seeder. In your case that would be 240". No skip and no overlap. Once in the field, you might need to check how true the machine pulls behind the centerline of the tractor and adjust the draft settings. If Omni XP is 3" to 5" accuracy, don't try to outguess the thing because if you set things 5" narrower and the signal varies -5", you'll then be off 10".

Sooner or later, you need to trust the magic of this stuff or not even have it. Might as well start trusting it now.