How to calculate Average Electric Field for a microscopic filed in dielectric?

In Griffith's book in The chapter of Electric Fields in Matter (The field inside dielectric) , He has considered the Sphere of Radius R and for the inner dielectric element He used the field as $$\frac{1}{4\pi\epsilon_0} \frac{p}{R^3}$$ My question is how can he tell that?...As The elements are not in R distance and how does you can tell that by averaging we can get exactly R?(https://drive.google.com/file/d/1LNKC1o9k5kSZFGcA_vV5FZ8pX-rMAu_r/view?usp=drivesdk)