Is it justifiable to leave when things are in the 'going ok' phase of an abusive cycle?
Yes, absolutely. You've got to remember that an abusive cycle is exactly that - a cycle, and the 'going ok' phase will not last.
Be safe. Leave while you can.
Definitely. It's okay to leave anytime you wish. That's your right. Don't feel bad about it or feel like you have to even tell the abuser of your decision. Do what is best for you - leave and the sooner the better.
Yes, and it's probably better for you than leaving during a fighting stage. If you try to leave while in a fight with your abuser, they will probably manipulate you into staying. However, if you blindside them while things are going well, they wont have the time to manipulate you, ESPECIALLY if you cut off contact. Always have a plan when you leave, that way very little can go wrong.