What's a normal share split?
There is no normal share split. However the combined total of the writer share must be at least 50 percent. You can only give yourself less than 50 percent if you're adding co-writers. But even then you need to ensure that your share and their share add up to at least 50 percent.
Other than that, it's up to the you and the other interested parties to decide the correct share splits for your work.