Fixed in GitHub in Fix and improve osm_mem_status Munin plugin by b1tw153 · Pull Request #794 · drolbr/Overpass-API · GitHub with some improvements to graph current memory grants for the base dispatcher and current/average memory grants for the areas dispatcher (which tracks a separate resource pool). Also, warning/critical thresholds are linked to the actual dispatcher limits with optional overrides.
You could try the new script if you like.
Just a small note, FairwayMapper.com has made available a pool of Overpass API slots to our own load balanced overpass infrastructure as a bit of a thank you to the OSM community. I am mildly pondering opening registration for a paid for service but thats just curiosity on my part at the moment.
Details available at FairwayMapper (note that you require 20 OSM changesets in the last 90 days to be eligible - and keys expire to free slots up if not used in the last 30 days).