The formula with 29 substituted
L/100km = 235.214583 ÷ 29 = 8.11
The constant is an exact definition, not a rounded factor: the mile is 1.609344 km, the US gallon is 3.785411784 litres and the imperial gallon is 4.54609 litres. The MPG to L/100km Converter explains where each comes from and gives a longer table.
The same 29 mpg (US) is 34.83 mpg on the imperial gallon, 8.11 L/100km and 12.33 km/L. If your 29 mpg figure came from a British source it already uses the imperial gallon, and the metric answer is 9.74 L/100km instead.
What 29 mpg (US) means in practice
29 mpg, 8.11 L/100km, is where several popular all-wheel-drive wagons and crossovers land on the EPA combined cycle. Front-wheel-drive versions of the same cars typically add 1 to 2 mpg.