/dev/shm default setting

Hi all, I am wondering what an ideal `/dev/shm` setting might look like. Let's assume we have a lot databases and need to define a shm size without knowing the database before hand. Is there any way to calculate it? For example the host system has 8GB of RAM and the default `/dev/shm` `tmpfs` has a size of around 4GB. Should the `--shm-size` be 2GB? The whole 4GB?

Thanks a lot for your input.