Ahh, that's right, the minimum on speed is 50%... (duty = 128)

So then you've got to increase duty by about 4.2 for every degree between 90 and 120 ...

How about something along these lines:

If temp < 90 then duty = 0

If temp > 120 then duty = 255

If temp > 89 AND temp < 120 then duty = 128 + ((temp - 90) * 42 / 10)

steve