Not sure you will ever be able to calculate a valid “Monstermark” for a ship with tubes. The ship benefits from taking no return fire while out of beam range.
And again, balancing capabilities is of limited value. When running Excalibur I quickly accepted she would never do the damage of any of the other ships, had no heavy ordnance to deal with fleets and could not come close to Lancer’s ability to take on single targets. And you know what? It didn’t matter. She just had to be the best she could be, the extra pair of hands that enabled other vessels to do more. But she was huge fun to run because you really needed the best out of every bridge station to maximize her effectiveness.