Well, first, the backoff algorithms are usually at the MAC (Medium Access Control) layer, so it depends what medium (thin Ethernet, UTP Ethernet, Fast, GigE, 10Gig, Wireless, etc) you're talking about. When I was taught this subject years ago, the backoff algorithm used by Ethernet was exponential, starting maybe at 2 clock ticks or something. (Then 4, then 8, then 16). But maybe there was a random fuzz factor involved. (If there is, then computing the average would be to compute the mean of the random range, and add that to whatever).