What strategy assessment did you use initially?
I don't know that there is an absolute formula, but I have heard that if 80% of your students show a need, that's a good whole class strategy lesson. Sometimes you will work on the same strategy in whole and small group, but sometimes it will be a different one, depending on the needs of the students. For example, you might introduce check for understanding as a whole group strategy but emphasize chunk letters and sounds as a small group focus.