It has not been standardized. But for distillation, Essential oils contain substances with boiling points up to 200°C or higher so keeping the temperature below that is desirable. generally, 60 to 160°C is prefered. When it comes to Mandagni we consider up to 120-250°C, this is specifically in the context of kupipakwa rasayana. As there is no specific standardization available for Dhoomagni you can consider this by keeping your objective of Arka(Distillation ) in mind.