Though I regularly use a spring dynamometer to set the thread tension of my Singer 201, I had major problems in finding the reason for top tension variations in a range of a factor 2. But finally I discovered the problem related to the design of the -|+ cup of the tensioner. Then I made this video about it. I think the same cup design is used on the Singer Featherweight and the 301. It may be by a case to case basis how much problem this design may cause problems for you. But the fix is easy:
Link to youtube video about tensioner fix
I could not have solved the problem without being able to measure thread tension. It very easy to do with a cheap spring dynamometer, and I think you already know them from school. I know that wrong thread tension cause problems for a lot of sewing machine users, so I recommend to get one or two of them:
Link to youtube video about spring dynamometers