Multiple SCSS equations to work out width minus margin - sass

Question

I have a mixin that splits items of a users choosing into an inline-block grid. Users can specify how many in a row, gutter and the child element to target.
This all works fine and an example is below:
Original - SCSS
#mixin list-grid($per-row, $spacing, $selector){
margin: 0 em(-$spacing/2);
#include clearfix;
//negate the display-inline biatch
letter-spacing: -0.31em;
> #{$selector} {
width: 100% / $per-row;
font-size: 16px;
position: relative;
padding: 0 em($spacing/2) em($spacing) em($spacing/2);
display: inline-block;
vertical-align: top;
letter-spacing: 0;
background-clip: content-box;
}
}
My question is.. When i try and use margin I need to rework the calculations as previously this worked because padding is not accounted when working out widths, whereas now with margin I need to adjust the widths to account for the chosen margin.
I'm not sure how to best approach this. Any help appreciated.
New Attempt - SCSS
#mixin list-grid($per-row, $spacing, $selector){
margin: 0 em(-$spacing/2);
#include clearfix;
//negate the display-inline biatch
letter-spacing: -0.31em;
> #{$selector} {
/* not sure what to do here */
width: 100% / $per-row -$spacing;
font-size: 16px;
position: relative;
margin: 0 em($spacing/2) em($spacing) em($spacing/2);
display: inline-block;
vertical-align: top;
letter-spacing: 0;
background-clip: content-box;
}
}
The latest attempt ends up with a width of 8% if I do a row of 3, with 25 gutter. My math must be way off.
Working example