Ok,
in case somebody requires something similar:
html\com_k2\templates\default\category.php
change:
<div class="itemContainer<?php echo $lastContainer; ?>"<?php echo (count($this->secondary)==1) ? '' : ' style="width:'.(number_format(100/$this->params->get('num_secondary_columns'), 1)-0.1).'%;"'; ?>>
to whichever suits you, e.g.
<div class="itemContainer<?php echo $lastContainer; ?>"<?php echo (count($this->secondary)==1) ? '' : ' style="width:'.(number_format(100/$this->params->get('num_secondary_columns'), 1)-3.1).'%;"'; ?>>