As Fowling's attorney, you should aim to get her the best possible deal in this contract negotiation. To do this, you should focus on getting her a higher percentage of the film's budget, a higher minimum guarantee, and a longer option period. You should also try to get her a higher percentage of gross profits and a bestseller bonus based on the New York Times children's and fiction lists.
In terms of the purchase price, you should aim for 3% of the film's budget, with a minimum guarantee of 2.5%. This will give Fowling a good amount of money regardless of how the film does. If the film is a huge success, she will still be compensated well. However, if the film does not do as well as hoped, she will still receive a minimum guarantee that is fair.
In terms of the option period, you should aim for a 6 month option, with the possibility of two 3-month extensions. This will give the studio enough time to get the film made without delaying too long. You should also try to get Fowling a per-week bestseller bonus, which will compensate her for every week her book remains on the New York Times children's or fiction list.
In terms of creative control, you should aim for Fowling to be a consultant and producer on the film, with final approval on casting and the script. This will give her some control over how the film is made, while still allowing the studio to make the final decisions.
Overall, you should try to get Fowling the best possible deal in this contract negotiation. By doing so, you will ensure that she is fairly compensated for her work and that the film is made in a way that she is happy with.