How do i negotiate a promotion?

I have been working in manufacturing with this company for 12 years. I put in an application for a management position, and after two interviews we negotiate monday. I am at a loss for how to handle any parts of this negotiation. I am currently paid hourly/manufacturing and would be going to salary/management. Looking at last years tax returns i made 60k, some of that with the help of overtime. Im not sure how much increase in salary i should expect or ask for with the new responsibilities i would take on. In my second interview, when i asked about my expectations i said 85k. I didnt receive any negative feedback. So how do i negotiate for a job at a place i already work for? Can you imagine negotiations breaking down, i dont take the offer or they rescind the offer, then i walk back out to the shop floor to work like nothing happened? How in the world do i handle this situation?

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