What is the rule for setting a node into "nowrite" mode

We had a disk-related issue on our centera and, during the troubleshooting of this issue, EMC support placed six of our nodes into a "nowrite" status.  We asked for an explanation as to why this was done and received two answers:

1. As for every node there is a threshold limit which has to be maintained so that the node should work fine. If the limit goes below 20% the node may go offline automatically
   
2. I dialed again to the cluster and found that the used capacity on all those  nodes which were set to “no write status” as true has gone above 90%(which is the permissible limit).If any of the nodes goes above 90% used capacity there is a possibility the node may go offline and it may create a Data unavailable status
   

I have asked EMC for further clarification on these explanations, but also figured I would ask this forum:  is there a "rule of thumb" for when a node should be placed in "nowrite" status?

The explanantions provided so far concern me because we have a mixed node centera cluster and the nodes (gen3, gen4, and gen4LP) all have different storage capacities, so using a % rule could leave significantly more space unused on the larger capacity nodes.

Thanks.