You might want to check the NADE (now NOSS) Self- Evaluation Guides. There is an entire section on tutoring which does not include a specific formula for tutor to student ratio. As others have stated, that is exactly the point. The number of tutors per student depends on so many factors that it is not possible to try to reduce that to a numerical equation. What you will find in the Guidelines can help support an argument for sufficient resources, both human and material combined with the need for clear programmatic goals and measurable outcomes to help you determine if you are meeting your students' needs.
Roberta Schotka, Director of Programs, PLTC
CRLA Certifications Director