FT doesn't have tools for calculating the current percentages based on the current world state, sorry, just for defining that during the initial world generation.

The only thing that I can suggest to get just the world data exported without any surrounding data is to zoom to the width of the world and check the "Keep Main Window Proportions" checkbox on the "Image Save Parameters" dialog. That's assuming that your projection is a squarish one. For rounded ones like Hammer, saving as a PNG will make the background areas transparent, if I recall correctly.