What I always use is a chaser with asymmetrical timing.
Before show time, I define the utmost fog a venue can have. So, basically, create a mist—way too much for the venue. Remember that ventilation and open (closed) doors may influence this behaviour.
Now, this said, I have the 'output fog' setting fixed in a scene. In general, I set the fan speed to a value that will reach the other side of the stage and get an even mist.
With the help of a zero-fog scene, I have a chaser with two steps. The off-scene and the 'output fog'-scene. Both share the hold at the same time.
With the help of a Speed Dial widget—this is what you are after—I can dynamically set the amount of fog I'm producing, whether slow pulses or fast pulses. The fan speed stays constant.
https://docs.qlcplus.org/v4/virtual-console/speed-dial
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