I have a man-in-the-middle digital rig, works off a 1/2" drive (adapt to any drive size you want), accurate to 1/10th of an inch-pound (according to its docs, I haven't tried to calibrate it to prove that). I can torque with allen hex stock, sockets, crows feet, tubing wrenches & screwdrivers with it. Not expensive either.
Now I've rebuilt diesel power units, auto engines, bulldozers, etc over the years. Torque is a bit dicey: galling, worn threads, rust, teflon tape, machine oil, or stuff like never-seize ( a metal rouge & lube compound)- those all change the torsion load. I've stripped aluminum heads rated to 20 ft-lbs at 14! I don't have a better answer but having some sense of what might be changing the thing you're doing to make it off-spec is a skill and an art.