First, don't think of the white surrounding areas as 'wasted space'. That's a mistake in my opinion. Much art is about studying the empty space that surrounds or lies between focal point in images, and sometimes the empty space can even be more important.
I usually print 6" square on 8x10" paper (roughly equivalent to you 18x24cm). That leaves 1" on each side, and I use 1.5" on top and 2.5" on the bottom for a top weighted placement.
To me that looks fantastic when you hold the print in your hands.
I do the same on 11x14" paper, where I print 8x8" square, and on 16x20" paper where I print 12" square - with similar top weighted placement. To me that looks fantastic, and all that empty space supports the image nicely.