This depends.
Are they an experiened founder who either have already taken money or have a term sheet? If this is your first venture, and they have the investors and experience, this is fair in my opinion.
On the other hand if they are a newbie like you, id say it should be 30%-40%.
Its true that finding a technical founder is hard, however its even harder to find a partner with a proven track record and real access to capital.
Selling transactions.