You don't have to work with percentages, -crop works with pixel coordinates
-crop follows this format
-crop WxH+X+Y +repage
W=width of section you want (after your resize if desired)
H=height of section you want
X=initial start x position (upper left corner of section you want)
Y=initial start y position (upper left corner of section you want)
+repage removes the virtual canvas which stores the crop coordinates
see
http://www.imagemagick.org/Usage/crop/#crop_repage
If you leave off the +X+Y, IM tries to generate all non-overlapping subsections of size WxH
see
http://www.imagemagick.org/Usage/crop/#crop_equal
Note you can also crop relative to a corner or side - see
http://www.imagemagick.org/Usage/crop/#crop_gravity
So in this case, if you know you just want the right side of some WxH, use
convert image -gravity east -crop WxH+0+0 +repage result
or if you want percent (
http://www.imagemagick.org/Usage/crop/#crop_percent), then if the W=47.343%
convert image -gravity east -crop 47.343x100%+0+0 +repage result
But pixels will ensure you get exactly what you want.