Hey GranniesG- I have a Sweet Sixteen with the digital tension upgrade. The way I keep track is pretty simple.
I have a notebook that I keep at my machine. I always get the bobbin case tension adjusted first for whatever thread I'm using in the bobbin. In my book I record what thread is in the top and what thread is in the bobbin. I make a note of what the digital readout was for the top thread once the tension is properly adjusted. This reading gives me a starting point next time I want to use that same thread combination in the top and bobbin. Next to the digital readout I make a note of anything I had to do to work with the top thread- if it needed a thread net, if it needed to be on the horizontal rather than the vertical spool pin to behave, if I needed to skip a hole in the pre-tension bar, etc. I also note if the quilt sandwich is unusual in any way (such as double or triple batting for an art quilt, strange combination of fabrics, etc.) or if it is a standard quilt sandwich consisting of a single layer of cotton or 60/40 batting sandwiched between a top and back that are comprised of quilting cotton.
I make a lot of experimental fiber art in addition to regular quilts, and as a result, I tend to use lots of different kinds of threads, fabrics and battings. Sometimes I will put a different thread in the bobbin than what I've got in the top. Having the tension notebook saves me some time by helping me start out in the general ballpark so that I can pretty quickly do a bit of fine tuning on the tension and be up and running.
Hope that helps.
Rob