Each degree represents 60 miles so 5 degrees equals 300Nm in distance. So if 8cm = 300Nm then it also equals 556.1Km or 556100M and then of course 55610000cm.

If you divide that by 8 then 1cm=6951250cm then multiplied by the cosine of the latitude (as the earth is curved etc) and you get that 1cm=3475625cm or put slightly nicer 1:3,500,000.