As for cutoffs, just look up max healthy forehead temperature, maybe 37.5. More important is to have prominently available hand sanitizer pumps and encourage people to use it before and after the event, and remind them not to touch their faces.
Maybe you've already done this:
Write a list of the names of everyone that attended. This way if any attendee turns out to have been infected you have a better chance of containing it within some section of your community.
I did run the meetup and all tested people were under 37 degree's and thus likely safe. Outside it was quite cold and it seems like the measured temperature on the head with the contactless thermometer. Measuring people right after they enter the room seems pointless.
The one time I tried it the person measure 32 C. 10 felt minutes later they measure 35.9 and another 20 felt minutes later they measured 36.4.
https://www.ncbi.nlm.nih.gov/books/NBK263237/
" Compared with oral temperature, Chue et al reported a mean difference ranging from 0.05°C (95% CI 0.01 to 0.08) to 0.12°C (95% CI 0.07 to 0.17) "
from the conclusions, they report a high false positive rate, making it inappropriate for some purposes. However, for this purpose, false positives are preferred to false negatives.
I brought the oral thermometer along with the forehead thermometer to be able to check people who test positive with the forehead thermometer with the oral one.
This Sunday I'm co-hosting a SlateStarCodex meetup in Berlin. There currently seem to be a handful of cases in Berlin, so I'm wanting to take reasonable precautions.
I don't think canceling the event is reasonable at this point in time. At the same time I do want to have a policy of keeping potentially ill people away.
I have ordered a contactless thermometer. Most were sold out at Amazon.de, so if you want to hold any events in the coming months, now might be the last time to order. I also got normal mouth thermometers.
But I don't know about what values would be good cutoffs for fever testing.