I have a 1969 Gibson B25-12n with a tailpiece that I want to have re chromed. It has always been tuned 3 frets lower than standard E tuning. My question is , how much should I loosen the truss rod, if any, while the tailpiece is off getting chromed ?.
So you'll be bringing it up to standard pitch? There's no magic formula as to "how much" loosening or tightening to do....it will all present itself when you string the guitar up and start to do your set-up.
You might want to start (before stringing) with a generally slack rod, then let the neck tell you what it wants. As always with truss rods, everything's done in "baby steps" and give the neck time to adjust anytime you change the rod tension.
OK... I get it. No, sitting unstrung for a week or so shouldn't change anything. In any event, it's probably a good time to clean and lube the rod & nut just to make sure everything's working right, then leave it "almost slack" (just a hair cinched-up, not totally loose) when you start to re-string and do your setup.
You shouldn’t have to loosen the truss rod.