The following is all conjecture.
I'm not seeing any corraletion between distance and Resistance. On our Run yesterday the most resists where had by one of the ranged, however the least resists where also had by one of the ranged. I know the Knock back eefect has a 20 yard range but otherwise the damage seem raid wide. One thing to be mindful of is that each time Rhyolith Steps on a volcanoe he gains a buff called "Molten Armor" this increases his damage by 2% per stack, If your stomping out all the volcanoes, not just the ignited ones you can stack this quite high aslso aslong as an ignited volcanoe is active it will be stacking "Eruption" on your raid, which stacks to 14 and you can have up to 3 stacks (of 14) of it at any one time in 10 ma. Each stack increase your fire damage taken by 10%.
Combined these two things can make healing impossible. We've found it best to ignore dormant Volcanoes and focus on getting he active one done as fast as possible. this does mean P1 is quite long, but we also go into P2 with only 5 or so stacks of Molten armour and minimal stacks of Eruption. We then all stack up on our hunter (cept the tank), since he's the only person with a range issue, and and Nuke, we pop raid CDs for the Stomps but otherwise we take it to the face.