Okay - it is sewing more reasonably now, without the regulating nut needing to be almost off the stud. The bobbin shuttle is set at about 1.5 oz (40g) to "pull the thread" vertically up from the bobbin. I found that little gem on SingerSewingInfo on their technical page about tension adjustment and calibration (
https://www.singersewinginfo.co.uk/tension/)
Then I played with the top tension. A hint I picked from Andy Tube (the last three videos in this little playlist
https://www.youtube.com/watch?v=83o6...97NsW5GYM7KD9U) was to make sure that when the eye of needle pierces the fabric (on the down stroke) the check spring comes to rest on the stop and also to make it so that the check spring itself is positioned (where it is held in place at the back of the stud) so that when the tension is fully engaged as you sew the stitch the check spring points pretty much straight up.
My check spring seemed bouncy enough, all right, but it was not resting on the check spring stop when the needle pierces the fabric - it was stopping still a wee bit above the rest at that part of the needle stroke (solution, adjust where the stop is positioned to match where the spring is when the needle enters the fabric). Once I did all those things I could get the regulating nut right on to the post and was able to make little micro-adjustments to the tension like you'd normally expect to do for a nice balanced, "locked" stitch where 90-degree turns look pretty sharp on both sides of the stitching. I did all the testing on a medium stitch length.
I hope that helps someone!