4129R,
Do you have a pic of your adjustment setup?
I would have to take a rear wheel off and it is cold and wet outside tonight.
Take a 1/2" UNF nut and drill out part of the thread with a 13mm drill to give clearance over the 1/2" cable outer.
Cut a 3mm slot in a flat on the nut with an angle grinder, to make the nut C shaped, so it will fit over the cable inner wire.
Pull the cable outer out of the trailing arm. Put the nut over the cable inner to space the outer away from its retaining C bracket on the trailing arm. You have now effectively made the cable inner 1/2" shorter on one side. You can do this on the other side to make the cable 1" shorter. For smaller spacers, either grind the nut down, or use a thinner nut.
I am surprised the cable outer is not threaded with a nut on it to do this with a fine adjustment. You can probably safely space out 3/4" each side without pushing the cable outer completely out of the C bracket on the trailing arm.
I find it much easier to adjust the cable outer on the trailing arm, rather than 2 x 1/2" spanners down the black hole, where there is only room for 1 hand.
When it is warm and dry, I can provide a photo. Remind me in warm weather.