If you're using writeMicroseconds() you put in the value you want within the limits 544-2400 (actually you can use numbers outside this range but it limits the actual pulse width to those).
If you want to know the mapping between angles in write() and microseconds look in Servo.h. I believe 0 = 544 and 180 = 2400 unless you have changed the limits when you did your attach().
And you can use any integer from 0 to 180 in a write(). The IDE example Sweep uses them all.
Steve