The answers come from the top left (time) section of the chart.....
CAP697 Section 4 page 15...looking at the 9 hr trip time....
at ISA +20....the 9 hrs becomes 8.6 hrs
at ISA -10....the 9 hrs becomes 9.2 hrs
SO, a 30 degree decrease in temp has increased trip time by 0.6 hrs
As a percentage....(0.6/8.6) x 100 = +7%