Hi Carl,
This is my opinion about the network traffic, in the old days there was some quotes saying that on a routed network the traffic pattern should fit in the 80/20 rule, which 80 is Unicast n 20 is Broadcast. it is old though!
there is not a certain rule or standard to measure the max. amount of the unicast, multicast and broadcast packets that should usually transit the networks. Rather it totally depends on the network's design and services you provide throughout your network.
to see if what you are experiencing is a normal behavior of your network or not, the first step is to identify which devices/services are involved in a broadcast or multicast storms. it could be either healthy RIP OSPF ARP SAP etc. or a product of a design fault or another type of misconfigs in the net.
therefore, to come to a solid conclusion, I suggest you use a decent network analyzer to identify, capture, and categorize broadcast and multicast traffic, identify its sources and then take proper action towards controlling it.
plz Rate if it helped.
Soroush.
Hope it Helps!
Soroush.