Don't! RL has priority.
Technically, cover doesn't give you a penalty. It gives a bonus to AC, which boils down to the same thing. But please don't substract from your roll, just to make sure it doesn't get counted twice. Also, I recommend that you don't make assumptions regarding the monsters' AC, even after you made the Knowledge roll. You might get nasty surprises (not in this case though).
The solifugids are not tall enough to be visible behind the altar, so they also have concealment.
(1d100)[84] - that's still a hit!
Ref saves
(1d20+2)[17]
(1d20+2)[20]
Last thing: As it evolves over time, can you please record the DC of the bomb Ref save on your sheet. Thanks!