[itsme]

you need to see the the documentation about your servo, see how much it can turn in degrees.
Oh, and remember when calculating this way: the starting position (the middle of the range) is half of the ratio. For example: if the ratio is 180, then 1 degree equals about 0.7 PWM, and the starting position is in the middle, witch is 90 degrees, witch is about 127 PWM