finally managed to get it to stretch to the edges with the following css syntax.
.wp-show-posts-read-more{
width: 100%;
max-width: 800px;
box-sizing: border-box;
}
.wpsp-read-more {
text-align: center;
display: block;
}
however the “Read More: button does not always align to the bottom of the cells.
do take a look at the site and you will notice one of the boxes have an alignment issue…