I replaced the margin-right: 6px which is originally on each columns by a transparent border. So that I can effectively define the size of the columns without calculating the width minus the margin...

Here an explanation of this tricks :

.column {
/* Remove the margin. */
margin-right: 0!important;

/* Put a transparent instead. */
border-right: 6px solid transparent;

/* Constraint the background stricly inside the borders. */
background-clip: padding-box;

/* When specify width, specify the width with the borders. */
box-sizing: border-box;
}