I do wish people who quote figures understood %s. And ones being paid to write articles should refund their fees when they get them wrong. You should be ashamed of yourself!
You say Tescos down 25%, FTSE 100 up 10% so there is a 35% differential. Tescos are down to 75% of their original value. FTSE is up to 110%, therefore the differential is now 110/75, i.e. 46.67%