Do the drop test. Put a fresh bobbin in the bobbincase, thread the bobbincase correctly except for the "pigtail". Hold the bobbincase up by the thread (leave a hand underneath so it doesn't hit the floor!), let it dangle and jerk the thread to test the tension. If the bobbincase drops a few inches and stops you are in the ballpark. If it doesn't drop at all it's too tight. If it hits the floor it's too loose and you forgot to put your other hand under it.
What is the pigtail referring to?