Each cable has a tolerance of plus or minus 1/8". It is possible to have two cables that are 1/8" short adding up to 1/4". Since there are 28 threads per inch, that quarter inch amounts to 7 threads, 3 1/2 on each end of the turnbuckle.
As for checking the stick-to-stop clearance, temporarily attach an angle in lieu of the cover while setting the tension and checking clearance. Two screws are enough through the angle if they go into the bearing bracket.