[Grml] Limit tmpfs RAM across mount points

What is the preferred grml way to limit total tmpfs RAM usage across all mount points. By default tmpfs takes 50% of RAM before swapout. We want a lower threshold of 20% RAM. With fstab syntax it's possible to limit one mount at a time, but is that the best one can do? It would be easier to limit the whole tmpfs subsystem and leave fstab entries "unlimited." Where is the 50% limit set, for example.

M