Severance offer, what should I ask for? - what worked for you?

I was offered 18 weeks of severance after 18 years as a forklift operator, and I have a few days to sign. I would rather hear the honest version than the polite one. I do not want to leave money on the table. Should I see a lawyer?

I asked for two more weeks and a neutral reference. They agreed to the reference and added a week.