It's most likely random between 20-24 for all thieves. The increase in HP/MP after a job advancement is still a mystery to many. I'm making an NL soon so hopefully I'll have the exact number.
I don't think there is an exact number. I watched some youtube videos of characters around 150 and all of them had different HP and MP. Nobody of them had washed since HP washing pre-bb was way too expensive for a regular player. They all had more than the min. HP and MP tho. It might be close to this calculation: Min. HP + extra HP @ 1st job advancement, 2nd ... , 3rd ... , 4th ... A nightlord will have at least 4.3K HP at 155 without hp washing while his min. HP is 3.4K HP.
