Keep it open the whole incubation. I have taken out he rubber in my incubators. You need to increase humidity at lockdown but ventilation is also important.
Good luck! Are you using local eggs? I have only been able to use shipped eggs and have not had amazing hatch rates. I think if you can keep the incubator in a room with stable temperature it helps as there is not much insulation on the j-12. I tried adding some as a test though and the temp went too high.