well personally if it were me, the answer would be none.
Check out the wiki article on Chlororothalonil:
According to the United States Environmental Protection Agency, chlorothalonil is a toxicity category I eye irritant, producing severe eye irritation. It is in toxicity category II, "moderately toxic", if inhaled ... Chlorothalonil is a Group B2 "probable human carcinogen"
Chlorothalonil is highly toxic to fish and aquatic invertebrates ...At a concentration of 164 Ã‚Âµg/l, chlorothalonil was found to kill a species of frog within a 24-hour exposure
[it is quite persistent in the environment] Chlorothalonil has been detected in ambient air in Minnesota and Prince Edward Island, as well as in groundwater in Long Island, New York and Florida. ...
The main breakdown product of chlorothalonil is 4-hydroxy-2,5,6-trichloroisophthalonitrile (SDS-3701). It has been shown to be 30 times more acutely toxic than chlorothalonil and more persistent in the environment
There are plenty of organic, non-toxic ways of dealing with fungus problems on your plants.