Hurricane Rotol mixture controls?

[klem]

The word "auto" relates to the engines ability to maintain the correct Rich mixture with changing altitude. However in CoD I do seem to be able to Lean the mixture off until the flame changes from Yellow to Blue, usually around 25-30% travel so either its modelling adjustable rich/lean level between the extremes or I'm simply hitting a crossover point.

I haven't found anything in the Pilots Notes that talks about gradual adjustment of Mixture, just fully back (Rich) or fully forward (Lean)