SINGER 66 stitch regulator guide, see here for stitch length to knob-turns.
This was checked several times by sewing on lined paper and measuring the setting of the regulator to stitches per inch.
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One turn is 360 degrees, a half turn is 180 degrees on the knob.
I don't know about you guys but once I wrote this down I was extremely happy! At this point though, as I'm sure some of you have figured out, after a while it's not necessary to know how many turns. You just start to know.