Predicting dew formation is a complex subject, but here is some relevant experience:
I use a hood on my ASA10, more for stray light exclusion than dew protection, but it does help a little.
The fan is a help, and I control the speed, depending on the conditions. It is best not to use it if possible.
I regularly encounter very high humidity - often 80% and above. I have successfully imaged when humidity is >90%. My record, at the end of a run I really wanted to complete, was 99%. The alarm on the humidity sensor was flashing and all the outer surfaces of the gear were very wet, but not the mirrors!
I don't make a habit of this, but I would expect that you'd be OK up to 90% RH. Just keep everything cold and run the fan for some time after you finish.