dividing face in only one direction
 

How do you do it?

The Polygon Subdivide tool works in both axes AND only divides in an even number of subfaces.

If I could snap the Split Polygon tool to fixed percentages of the edge...

It's possible using magnets but it's very boring: you have to decrease the number after each split

Have you tried the Cut Faces Tool? You can't snap it to the grid or anything but at least you can constrain the cutting angle to 45 degree increments. It can also operate on selected faces only.

you can snap it to a grid.

After you cut, select the history item and use the Show Manipulator Tool to reposition where the cut was made. You can then snap it to the grid, if you wanted.

You can also use the Split Polygon Tool.

Good to know :) Anyway, I've just selected the newly created verts and used the move tool to snap them where I want...

I found another way to divide evenly a rectangular face:

You split the two lateral edges using the Split Polygon into 7 pieces, for example, and then Split tool again the vertices created.

It doesn't work in polygon proxy mode tough.

it does work in poly proxy mode. just ignore that maya complains about subd creation. once you finished splitting the faces, the error message will be gone.