I had a little design situation come up where I was making a fluid grid of boxes with floats. I wanted to specify how many boxes across a row was very easily, and have them flush against both edges of the container. Not too difficult, as we know from not overthinking grids and using the right box-sizing, you can get four floated boxes in a row width 25% width - easy.
But what if you want to use margin between the boxes? Still totally possible, just requires some thinking. Say you want four in a row, you'll need to figure out how much space you have left after all the margin is used. Because you don't want margin on the last one in the row, that's 3 margins:
100% - (3 * MARGIN)
3 is really "rows you want minus one", so:
100% - ((ROWS - 1) * MARGIN)
Then you divide the space you have left by how many boxes you want on that like, so: