How much weight should a single axle tractor hauling a 26 ft tandem axle trailer be tagged for?

See the related link to "Bridge Formula Weights Calculator". Although a set of tandem axles is allowed 34,000 lbs. on the Interstate, this is reduced when you get below a bridge length of 35 feet between the trailer and drive axles.

If you don't want to measure out your distances between axles, at a minimum, you need to tag it for 67,000 GCWR.