I'm actually working on some code that can automate this, but it's not quite done.
For a 901 paladin on Krosus... I'd set the Mythic+ level to around... +5 probably, and then use an ilvl range from 895 to 925 for now, that should give decent results I'd think. This is assuming that you are using Holy Shield and Blessing of Spellwarding talents, which make you much tougher on that fight. If you are not using Seraphim and you are using Bastion of Light, you may need to go up to Mythic level +6 for it to be threatening enough.