I'd think it depends on which CFM blower you're using, Neil, as well as your blower damper settings - For my 22.5 WSM, I have a 10 CFM blower with the damper closed/open halfway; all lower vents are completely closed (save the blower vent, naturally), and the top vent is cracked open about the width of a dime, or perhaps just a tad bit more. That's my setup for running low and slow, and I've enjoyed considerable success with it; if you want to go with higher temps, you may have to modify that configuration accordingly to allow more oxygen in.
Unless someone using your exact same ATC device model, CFM output, and WSM size can speak specifically to your needs, I'd suggest trying out my configuration and changing up the other settings if or as deemed necessary.
Regards,
Rooster