Royalty arrangement

I’ve seen a few threads about freelancing, but none that deals with this that I can recall.

I’ve got an offer for designing a food package with added function. It’s an interesting concept and could result in a patent. The client has mentioned a royalty arrangement, but no numbers yet. So my question is - what do I need to know, what do I ask the client, and how do I not get screwed? Who should word the contract - me or him? What is a normal number and what is a slap in the face?

Normally I would calculate how much I need to get paid for my time, and write up an offer, but now I’m not sure. But also I’m fine with royalties and nothing up front if I can work at a much slower pace (have a full time job).

Any advice appreciated.

do a search for royalty. There are several topics on this, might be some beneficial advice already in there.