I always check my B/C gap with the cylinder pressed towards the rear, that allows me to have a good idea of the B/C gap if a shim is needed.
When checking for end shake, I would push the cylinder forward and check the gap again. If the cylinder is hitting the barrel, add shims until there is some clearance present and then re-measure the end shake and add the stackup of shims to the remaining dimension for end shake.
As for a target for these dimensions, my preference is end shake between 0.002 and 0.001 inch and a B/C gap between 0.008 and 0.005 inch. As for ideal, that would be an end shake between 0.0005 and 0.001 inch and a B/C gap right at 0.005 inch, however ideals are only very rarely met. In addition, what you are shooting is a factor on hom much end shake is acceptable. In a 22 caliber there is so little recoil that end shake of 0.003 to 0.004 inch really isn't worth worrying about, however in a 44 Magnum that same amount of end shake will cause a rapid increase in the end shake do to the cylinder behaving like a slide hammer. For the hard hitting calibers, it's best to spend the time to get the end shake as close to zero as possible without causing the cylinder to bind.
It's also a good idea to close the yoke without a cylinder installed and check to see if the screw that retains it eiher needs replacing or the end peened slightly. Ideally you want just a slight hint of drag when opening an empty yoke.
