I replied to your comment on the elanthipedia talk page, but I'll add a synopsis here.
1. Cast with enough mana to prevent loss of conversion. I remember being able to endure the full duration around 45 mana. You might be able to get there with the right alignment and SAP.
2. The attunement gained is fully dependent on your spirit pool size. Two examples:
*20 charisma, 20 discipline, 20 wisdom, 20 intelligence = 100 spirit pool (90% restores 90 mana. with a 30 cast, effective 60 mana gain)
*50 charisma, 50 discipline, 50 wisdom, 50 intelligence = 250 spirit pool (90% restores 225 mana)
https://elanthipedia.play.net/Talk:Eylhaar%27s_Feast