Thanks for this thread!! It helps us both.
Does the use of the machine cause the bobbin tension to slowly change? I find from time to time I have to tweak it. I usually use the machine in question for piecing so the tension setting should not change. And yet it does, from time to time. Perhaps over months. I wonder if the sheer vibrations of the machine can loosen/tighten the tension screw in the bobbin case.
If it seems I have to really turn the dial a long way on the upper tension, I take out the bobbin and do the yo yo test. Sometimes my bobbin has gotten tighter. I loosen it just a tad and I can put my upper tension back in the neighborhood where I think it should be.
(I have notes of where I get good stitches for piecing on my upper tension dial.)
On a related note, how much should one change the bobbin tension? I usually limit myself to 1/4 turn. Is that the right range?