Thank you! More good info to add to my notes.The humidity spike with 99.5 temps can lower Oxygen in the incubator. To help with that problem, lower the temperature to 98 degrees around day 19.
Vent open is good and on my brinsea it is a control slider that opens and closes the vent.
Humidity during incubation is important for air cell development. 30 to 35% is usually good but you may need higher humidity for light eggs--like Dorkings for example or silkies. The key is to watch the air cell and use is as a guide. Some weigh the eggs and come up with an average weight loss. If Humidity and air cell is as suspected problem, that might be a good thing to try.
I pencil-marked the air cell this last round and the ones that developed seemed close to where they should be; I'm going to try weighing the next batch.
I have the Octagon Eco - the temp is adjusted by a little screw inside the top - any ideas on how to lower temp the last few days without taking apart the bator?