http://wiki.arcadecontrols.com/index.php?title=Spinner_Turn_CountIf I go to that Wiki page, it says that Arkanoid has 486 counts per revolution. Using the formula, 486 * 100 / 1200 = 40.5, which should be the correct sensitivity setting in MAME.
The Wiki page also states that 120-130 degrees will move the Vaus from one side of the screen to the other. However, in my testing, including on a machine at a barcade last night, the Vaus seems to move all the way across the screen with a full half turn of the spinner, so 180 degrees.
I have my Turbo Twist 2 spinner sensitivity set to 30, which feels totally correct to me compared to the original machine and, indeed, moves the Vaus from one side of the screen to the other with a full half turn.
Changing the setting to 40 definitely feels too sensitive. This is all subjective, of course, but I would be curious to see a list of everyone's preferred settings for spinners and trackballs and see if people go mainly by what feels right, by what feels authentic, or by math based upon the Wiki numbers.