PLEASE HELP! Ghostwriting percentage help

I will be writing an autobiography for someone who is a friend.  She and I have agreed that my fee will come exclusively through back end royalties.  Yes, I know all the warnings against this, but I have my reasons and I trust her.  We have a contract to ensure the project will be finished and produced (likely self published).  She has publicity connections that will probably guarantee about 10k books to be sold.

Past that, what percentage of profits should I charge for this? I will be doing all of the writing and most of the research.  She will handle publicity.  Since I am not being compensated up front, I would assume that I could ask for a higher percentage.

So what should I ask for 25%, 30%, 50%?

Thanks for suggestions about the percentage. Please don't comment about the arrangement, like I said this is a labor of love and I have no problems with it this way.  I wouldn't do this for just anyone.

Thanks all!

Brian