Severance offer, what should I ask for? - 26 weeks in

I was offered 16 weeks of severance after 16 years as an accounts payable clerk, and I have a few days to sign. I do not usually post, so bear with me. I do not want to leave money on the table. Did anyone negotiate and succeed?

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