The node-cryptonote-pool project computes Network Hash Rate as (difficulty / block_time_target), and uses a constant of "120" baked in to the source code in pages/home.html, which I believe matches the block time target of Monero. Iridium uses a block time target of 175sec, so I believe that the 120 should be changed to 175, making the lower number likely more accurate. That project doesn't really document everything that needs to be changed to port it to another coin.