Following the formula above if you want to get degrees Celsius you have to subtract 32 from degrees Fahrenheit multiply the result by 5 and divide it by 9.
So in case with 8 we have: ((8 - 32) × 5) ÷ 9 = (-24 × 5) ÷ 9 = -120 ÷ 9 = -13.333333333333 °C