I keep my breeding rats in a pet style setup/treatment as my personal preference. My small group now lives in a 40 breeder that's 36x18x16. I'm making plans to add additional levels that can be closed off or opened from other levels depending on need.

The engineering is easy, but I'm not sure what the appropriate height should be for each level. I want them to have enough headroom for comfy lives but don't want to waste any space if the rats won't benefit. The height matters when it comes to the hardware cloth I'll need to buy and will effect the number of levels I make.
So how tall is tall enough?