Sass OO Grid

_base.scss

// --- Variables
// ---------------------------------------------------

// grid
$gutter: 10;
$max-width: 1100; // Max grid wrap width
$sm-column-count: 4;
$med-column-count: 6;
$lg-column-count: 12;
$mqs: 380; // small breakpoint
$mq1: 476; // Mid range breakpoint
$mq2: 600; // Med screen breakpoint
$mq3: 870; // Large screen breakpoint
$mq4: 1024; // xlarge screen breakpoint

// --- end variables ---------------------------------

// --- Mixins
// ---------------------------------------------------
// Calculate small screen grid based on $sm-column-count
@mixin sm-grid($columns) {
  width: $columns * (100% / $sm-column-count);
  padding: 0 $gutter+px;
  padding: 0 $gutter/$base+rem;
}
// Calculate med screen grid based on $med-column-count
@mixin med-grid($columns) {
  width: $columns * (100% / $med-column-count);
  padding: 0 $gutter+px;
  padding: 0 $gutter/$base+rem;
}
// Calculate large screen grid based on $lg-column-count
@mixin lg-grid($columns) {
  width: $columns * (100% / $lg-column-count);
  padding: 0 $gutter+px;
  padding: 0 $gutter/$base+rem;
}

_gird.scss

// Flex parent container
.flex {
  @include clearfix;
  position: relative;
  clear: both;
  padding: $gutter/2+px $gutter+px;
  display: block;
  max-width: $max-width+px;
  max-width: $max-width/$base+rem;
  margin: 0 auto;
  
  // Grid class objects
  @for $i from 1 through $sm-column-count {
    .g1-flex#{$i} {
      @if $i != 3 {
        float: left;
      }
      @include sm-grid($i);
      
      &.push{
        float: right;
      }
    }
  }
  .g1-hide, .g2-hide {
    display: none;
  }
  .g1-show {
    display: block;
  }
  // Adjust padding for nested items
  .flex-nested {
    
    &:first-child,
    &.g1-flex4:last-child {
      padding-left: 0;
    }
  }
  
  @media all and (min-width: $mq2/$base+em) {
    // Grid class objects for med screens
    @for $i from 1 through $med-column-count {
      .g2-flex#{$i} {
        @if $i != $med-column-count {
          float: left;
        }
        @include med-grid($i);
        
        &.push{
          float: right;
        }
      }
    }
    .g2-show {
      display: block;
    }
    .g2-hide {
      display: none;
    }
    // Adjust padding for nested items
    .flex-nested {
      &:last-child {
        padding-right: 0;
      }
      &.g1-flex4:last-child {
        padding-left: $gutter+px;
        padding-left: $gutter/$base+rem;
      }
      &.g1-flex4:first-child {
        padding-left: 0;
      }      
    }
  }
  @media all and (min-width: $mq3/$base+em) { 
    // Grid class objects for large screens
    @for $i from 1 through $lg-column-count {
      .g3-flex#{$i} {
        @if $i != $lg-column-count {
          float: left;
        }
        @include lg-grid($i);
        
        &.push{
          float: right;
        }
      }
    }
    .g3-show {
      display: block;
    }
    .g3-hide {
      display: none;
    }
    .g3-offset1 {
      margin-left: 8.4%;
    }
    .g3-offset2 {
      margin-left: 16.8%;
    }
    .g3-offset3 {
      margin-left: 24.2%;
    }
  }
}

I am supposing that you are using * { box-sizing: border-box; } since you are not using the gutter to calculate the widths, is that right?

@kbariotis Yes, that's correct.