score:5
After looking at lots of atlases and physical geography books on the internet archive, the first mention I found of 70.8% water and 29.2% land is in the German Knaurs WELT-ATLAS, 1932 edition.
Even more precise numbers are given by the book Mother Earth by T. A. Ryder in 1930 (so published two years earlier). It says 70.78% water and 29.22% land.
Most books between 1924 and 1932 don't give an exact number, they just say something like 'nearly or almost three fourths of the earth's surface is covered in water'.
It looks like some authors were behind current knowledge (or maybe the editions weren't updated?). For example, one book after 1930 says 72.5% water and 27.5% land (Earth Lore A Physical Geography, 1932).
I haven't found anything close to your numbers before 1930, but the 1924 edition of Bartholomew's The handy reference atlas of the world gives areas which work out at 71.8% water and 28.2% land.