We could add a screen for cache which tracks:
• db.serverStatus().wiredTiger.cache["bytes currently in the cache"]
• db.serverStatus().wiredTiger.cache["maximum bytes configured"]
And could maybe this to show rate at which cache is dirtied:
db.serverStatus().wiredTiger.cache["tracked dirty bytes in the cache"]

so, you just calc the % from that,
maximum bytes configured, two percentages, "% dirty" and "% used", dirty will fluctuate, used should just grow til max.