How do you calculate the incubator's volume needed to incubate your petri dishes?

Anyone knows a good rule of thumb to calculate the volume needed for incubation of petri dishes?

I'm in the process of developing a QC microbiology lab for a pharmaceutical company and I need to buy incubators that could handle 300 petri dishes at a time. I've tried contacting companies, but none has a technician who knows how to help me beyond the information on their portfolio, which is rather frustrating.

Does anyone know a good rule of thumb to estimate the incubator's capacity by their volume and dimensions? I know there's a difference between total internal volume and actual usable volume after temp qualification, but no one could inform me of this difference yet.

Thanks in advance!