his aura is 30 yards, so yeah you need to judge the distance pretty well since heal max is usually around 35-40 yards -- some of the healing spells with 30 yard ranges will be worthless during phase1. Usually you'll want your caster pit in the middle of the platform, and then your tank/melee about 15-20 yards away from the platform. I don't have any pics of his room but once you find that sweet spot its pretty easy to keep that distance always.

There IS another strat, especially if you have a druid healer. Remember, the debuff only causes 300% casting speed for mpm=instants. So you use only instant heals and tank and DPS him on the platform - the damage taken on the platform is minimal, especially if you have disease totem/cleansing. If most of your DPS is melee this is great, if most of your DPS is casters this won't work. You still need to dance in phase2 either way. Also remember in phase2 there is no aura debuff, so you don't have to maintain 30 yard distance from him - just remember his platform erupts as well so you can't stay on it.