This 'math' is absolutely terrible, not to mention completely unfathomable; You can't just make up stats. To decide upon an average, you need to either do significant testing and then show what the average or total damage is, or (again after testing) work out what the actual chance of getting each roll is and the amount of damage each roll does.
Also assuming that the 'has no effect' message is indeed linked to whether or not the mobs level is a multiple of the random dice roll, then using ??? Holy on a level 120 mob will always do at least SOME damage. So it does have some good uses, and this will significantly boost the average damage. Unfortunately, this is only applicable to mobs with a level of a multiple of 60.
While your example shows 1 x good roll, 4 x bad rolls and 7 no effects, this won't always be the case, depending on the level of the mob. A lvl 100 mob will hit on 1,2,4 and 5, while a 101 mob will only hit on a 1.
It also requires further testing as to which number is actually the huge damage. If it's 1 then that's fine and dandy, but if it's 6 then it could imply that only mobs of a multiple of 6 can get huge damage and mobs outside that level scope it's pointless using it on.
It might of course not be based on level at all, but without significant testing on mobs with a known level, then we'll maybe never know (unless SE tells us).