When you have a book proposal and they ask you to propose a percentage of earnings and an advance- how do you calculate?
Like any other design project, you can charge on an hourly basis or a fixed amount depending on the scope and how much time the process will take to complete. I would start by setting delivery stages for the project, with that, calculate the amount of time it will require you to complete each phase. Define how much you want to get payed for each hour. Add up the total amount of hours and analyze from there if the total cost of the project is a reasonable amount that your client will be willing to pay.