How can I get the least disapproval from Alistair after saving Connor?

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

-21 should be lowest you can get,but i don't think 2 more points will do you any good.If you have savegame from earlier you could check this conversation or gifts if you missed something to get more points before doing the quest.

forum conversation

gift list

"Use him in your party, quite often. Make sure you give him his personal gifts, Duncan's Shield which can be found in the grey wardens stash in denerium, and his moms pendant which is in redcliffe castle. If you are in another relationship already, Alistair will kind of stay away from you, so make sure you break anything else off. In Redcliffe, getting the mages to do the ceremony to save Connor instead of Jowan boosts his rating quite a bit. Basically, playing a goody goody character, keeping him in your party and talking to him often, without putting him down or insulting him, boosts his rating."