On a guide I found the following set up for a Mage:

1. Self: Being attacked by a melee or ranged attack (Activate: Rock Armor)
2. Self: Mana or Stamina < 50% (Group Heal)
3. Self: Mana or Stamina < 25% (Use Lyrium Potion)
4. Self: Health < 75% (Heal)
5. Self: Health < 75% (Regenerate)
6. Ally: Health < 75% (Heal)
7. Ally: Health < 75% (Regenerate)

Is this a good approach?

As far I as know, the tactics are prioritized from 1 to downwards. So in this case, if this mage is always attacked, s/he will never be able to heal themselves, right?

Another question about the preset on the top left side. On wiki, it says that these presets will be used after customized instructions. I thought this was valid only for the top right the default behaviour. I believe the top-left presets are only to give templates to use or modify to our needs.

Thanks for the answers in advance.

Depends. For mages, I wouldn't set Morrigan up the same as Wynne, for exmple.

I don't know what your reasoning behind the no2. tactic is 2. Self: Mana or Stamina < 50% (Group Heal). Group heal isn't an ability that works well with tactics. It's probably best just to activate that one yourself. It's also costly, so blowing it when you don't need it just wastes mana.

Your tactics are going to depend on what role you have. Wynne's tactics are going to look a lot different than Morrigan's. Morrigan is a control mage, so your tactics are going to work on debilitating enemies. Wynne is a support mage so her tactics are going to cast buff and restorative spells on the party.

You've obviously got support mage tactics set up here, but it's missing the buff abilities like Heroic Aura and such. You may also want to consider the stamina regeneration spells to keep your party able to use their abilities.