Map scale confusion.

[feanaaro]

If your equator is 40225 km long, your planet is roughly spherical, and you are working on an A4 sheet, then 1cm =~ 1354 km. This is really simple and there is no need of making the planet a flat square or splitting it in parts or anything.
1cm^2 = 1 833 316 km^2, therefore 30 million km^2 =~ 16.4cm^2, or a little more than a 4cm*4cm square.
However, with all due respect, if given the km^2 size of 1cm^2 on the map you are not able to calculate how many cm^2 you would need for a given number of km^2, perhaps you should not worry about these things in the first place, and just mapping without bothering to consider scale (which is how humans have mapped the world for most of their history, anyway).

[lastofthemany]

There is an issue with the scale calculation above. If you are talking about a rectangular map of a spherical world, then the scale of 1cm = 1354 km is only true at the equator. The closer to the poles you get, the greater the distortion. In effect your ratio scale approaches 1to1 till 1cm = 1cm at the poles. That is because the top and bottom neat lines of the map are actually two different points, the north and South Pole respectively. To make a long story short, follow Feanaaro's advice. Scale is elusive on a global map that isn't an actual globe, unless you have a program that allows you to recalculate your map at different projections.