Bernina bernette 65 tension

Question

i cant get the tension right on my bernina bernette 65.
the manual suggests that the dial should be on 4 but it is too tight so ive tried 3 and 2 and in between.
the slightest movement either makes it too tight or too loose.
after a billion of test runs on every dial combination possible i finally found the best one but now after sewing for awhile the tension has gone and i cant get it right again.