Test Heat Before Sitting Over Steam.
Let it cool down until it feels relaxing like a warm bath.
It should be 100% comfortable and soothing.
(Note: If at any point you have the instinct to get up because it's too hot then you should turn off the burner and/or let the steam cool for a minute before resuming your session.)
If you are having trouble controlling the heat level, please follow the directions for the Mild Steam Setup.