Width Settings/Number Of Columns

Hi
Just wondering, we are trying to get a layout to suit our site, but everything appears to either left aligned or not set to use 100% of the page width.
Have tried the obvious, editing CSS setting in various places but unable to locate a fix for this, however we have found a code snippet that we cant locate anywhere.
“If we set grid number to 3, the setting states width=”26%” or if set to 4, the setting changes width=”18%”
Obviously these are the incorrect percentages as they should be nearer grid of 3=”33%” and grid of 4=”25%”. But we cant figure out where this snippet is being hooked from in order to reset it or if we are simply looking to deep into the code.

The snippet we have found looks like this:
.product_grid_display .product_grid_item {
width: 26%;

or

.product_grid_display .product_grid_item {
width: 18%;

Any input would be very gratefully received as we are having a lot of “white space because of this setting. ??
Many thanks in advance.

https://www.ads-software.com/extend/plugins/wp-e-commerce-grid-view/