The formula $T = \dfrac{\log 2 \cdot t_i}{3 \log(D_f /D_0)}$ is used by X-ray technicians to calculate the doubling time of a malignant tumor. $D_0$ is the diameter of the tumor when first detected, $D_f$ is its diameter at the next reading, and $t_i$ is the time interval between readings, in days. Calculate the doubling time of the following tumor: its diameter when first detected was 1 cm, and 7 days later its diameter was 1.05 cm.