I can claim some experience of dumping (fuel that is) having dumped on Hastings, C130's, and DC10's.
The general advice used to be don't dump near Cb's, don't dump in an orbit,,especially a descending orbit and try to do it above 6,000feet. Also, don't do it if you have an engine on fire (obvious) and avoid power changes.
I also had the experience of being dumped on when a local Thorney SAR chopper scambled to an overturned Hovercraft off Portsmouth. It passed directly over me at under a 1,000 feet with the stuff sewing out. All I and my garden on Hayling Island got was a strong smell of kerosine.