RC Column Design (ACI318) - How is the c/dt ratio calculated?

The variable r is used to denote the ratio of c/d_{t} where c is the depth of neutral axis from extreme compression face and d_{t} is the depth of tension reinforcement. The calculation starts by making an assumption about the value of r and performs an iteration until it manages to balance the calculated and required eccentricities. The method is explained in more detail in "Reinforced concrete" by Edward G Nawy.

Since it is an iteration, it does not show up in an equation as normally expected.