If I tighten the scotch tension so that the bobbin does not turn on the spindle I can't really control the "pull" and the fleece goes through the orifice too fast.
A: No the ratio is not set by the movement of the bobbin.
The ratio is set by three things (the first 2 are the most important) 1) the drive whorl that you are spinning with and 2) the amount of yarn that is drafted per treadle. 3) the way that the yarn feeds into the bobbin. If you consistantly allow a small amount of yarn to move into the orifice, the yarn will gain a few more twists as it makes it's way into the bobbin.
If on the other hand you do a long 20 inch draw and then allow it to wind on in a very quick manner there will be very little extra twist from that created during drafting.
You should never tighten the bobbin so that it stands still. On a single drive, scotch tension wheel, the bobbin should be free to turn the exact same speed of the flyer while your are drafting. But have enough braking tension so that when you feed the yarn towards the flyer the bobin slows down and pulls the yarn in. If the bobbin can not freely spin it sets up a whole series of difficulties that make spinning uncontrollable.