Being an ex-engineer, the terms "tight and loose", or "a good tug", or 1-1/2" are all about as inaccurate as, well you name it. Quantified forces and deflections are about all we have to go by, as a starting point at least. The Selden manual on rig tuning gives a pretty good description of how to indirectly measure stay tension by using a 2 meter rod clamped to a stay at one end of the rod. Then with the cable tension slackened, by loosening its turnbuckle, the free end of the rod, which is lying along the stay, is marked on the stay with a Sharpie. Upon tightening the turnbuckle, if the recommended stay tension is, say 15% of ultimate tension, then (correspondingly) elongation of the stay under 15% elongation from slack will show up as 3mm from the end of the measuring rod to the "slack" mark that you put on the cable. Different cable diameters are charted by Selden with corresponding elongations at 15%. Doing all of the stays in the same manner will give you at least a measured starting point, and you can thereafter count turnbuckle rotations during balancing adjustments. If you know what your turnbuckle thread lead is, you can calculate the ratio of turns to percent elongation, and get a feel for the forces that you have applied at the pier. The issue remaining is that we don't seem to have any recommendations of elongation in the stays to go on. Selden's recommendation of 15% is based on their masts. If queried, they may be able to provide some numbers from past knowledge of the Morgan 38's, or their own experimentation.
Until a stay tension can be "felt", it will take a fair amount of practice "calibrating" one's tug in comparison to the known tension. I remember doing a similar exercise in the Navy, when one did not have a torque wrench, or enough time to go get one! We got pretty good at applying the correct torques after some practice. Obviously, a seasoned sailor will have "the hand" without knowing what the numbers are, but until one gets there, some starting exercises probably would be good.
Rick, Xanadu