Recommended booking time for a tibet permit

Hi

I am planning to visit Lhasa on the 1st of May until the 4th of May. I current don't hold flights into Lhasa or a Chinese visa or even the tibet permit. I plan to book my Chinese visa in mid March.

Please could you advise:
1. What is the recommended approach for tibet? Can I book my flights now?
2. How many days before my trip can I apply for a permit?
3. Do I book flights first or get the permit first?
4. Are my dates likely to be impacted by tibet opening dates?