Based on the numbers given on the Dragon Age wiki, here's the super complicated way of minimizing the approval rating loss:
With Alistair in the party:
- Freeing Jowan: Convince Alistair this
was the right course of action to
avoid a loss (+0)
- When choosing who to enter the Fade, answer that "You must think about who will go" before making a decision (+4)
- When Isolde is sacrificed, use Cunning to reason with Alistair (-9)
- If you are a mage, enter the Fade yourself. Then, accept an award from the Desire Demon. The "Pleasure is empty. I want the love of those around me." option allows you to raise one character's approval. Use it on Alistair (+20)
- When Alistair confronts you at camp, pick the "I think that turned out quite well, don't you?" option to get (+7). On the PS3 however, this will result in (-13).
- As others have noted, you can lose up to (-41) in this conversation if you're not careful!
31 - 9 = +22 Approval (as Mage), +2 Approval (as Warrior/Rogue).
Without Alistair in the party (for all parts of this quest):
- If you are a mage, enter the Fade yourself. Then, accept an award from the Desire Demon. The "Pleasure is empty. I want the love of those around me." option allows you to raise one character's approval. Use it on Alistair (+20)
- When Alistair confronts you at camp, sooth/persuade him that you did the best you could. (-9), or (-6) if you convince him there was no other option.
20 - 6 OR 20 - 9 = +14/+11 Approval (as Mage), -6/-9 Approval (as Warrior/Rogue).
Hope this helps.
Source:
Dragon Age Wiki: Arl of Redcliffe Quest
Allowing Jowan to use Blood Magic may trigger Alistair to lose (-10) immediately, and during the Camp conversation, up to an additional (-41) can be lostcitation – tzenes Jul 8 '11 at 2:44