Because you are simulating, you can have each simulated node keep a count of the number of messages it sends and the number it successfully receives. Then at the end of the simulation, you can have your program run through all of those data structures and total up the values, and calculate the fraction.
You might want to use two counters on receiving, one for packets addressed to the node specifically and one for packets that are "pass through" to another node; likewise you would want two sending counters, one for self-generated messages and the other for pass-through messages. The overall system reliability is the fraction of the total self-generated messages relative to the total successful terminal receives.
