What thought consumes Darnay’s mind in his prison cell? (Book 3 Chapters 1-7)

Darnay understands why Dr. Manette was consumed with making shoes. In Darnay's case he chose to fill his mind with measurements of his cell. That kept his mind off his horrible predicament.

"Five paces by four and a half, five paces by four and a half, five paces by four and a half." The prisoner walked to and fro in his cell, counting its measurement, and the roar of the city arose like muffled drums with a wild swell of voices added to them. "He made shoes, he made shoes, he made shoes...."