quick flex layout

flex-layout.scss

@mixin flex {
  display: -webkit-box;
  display: -moz-box;
  display: -ms-flexbox;
  display: -webkit-flex;
  display: flex;
}

/**
 * 主轴对齐方向 box
 * @params flex-start flex-end center space-between
 */
@mixin justify-content ($justify) {
  @if $justify == flex-start {
    -webkit-box-pack: start;
  } @else if $justify == flex-end {
    -webkit-box-pack: end;
  } @else if $justify == space-between {
    -webkit-box-pack: justify;
  } @else {
    -webkit-box-pack: $justify;
  }
  -moz-justify-content: $justify;
  -webkit-justify-content: $justify;
  justify-content: $justify;
}

/**
 * 交叉轴对齐方向 box
 * @params flex-start flex-end center baseline stretch
 */
@mixin align-items ($align) {
  @if $align == flex-start {
    -webkit-box-pack: start;
  } @else if $align == flex-end {
    -webkit-box-pack: end;
  } @else {
    -webkit-box-align: $align;
  }
  -moz-align-items: $align;
  -webkit-align-items: $align;
  align-items: $align;
}

/**
 * 子元素的显示方向 box
 * @params row row-reverse column column-reverse
 */
@mixin flex-direction ($direction) {
  @if $direction == row {
    -webkit-box-direction: normal;
    -webkit-box-orient: horizontal;
  } @else if $direction == row-reverse {
    -webkit-box-pack: end;
    -webkit-box-direction: reverse;
    -webkit-box-orient: horizontal;
  } @else if $direction == column {
    -webkit-box-direction: normal;
    -webkit-box-orient: vertical;
  } @else if $direction == column-reverse {
    -webkit-box-pack: end;
    -webkit-box-direction: reverse;
    -webkit-box-orient: vertical;
  } @else {
    -webkit-box-align: $align;
  }
  -moz-flex-direction: $direction;
  -webkit-flex-direction: $direction;
  flex-direction: $direction;
}

/**
 * 是否允许子元素伸缩 item
 * @params {integer}
 */
 @mixin flex-grow ($grow) {
  @if $grow == 0 {
    -webkit-box-flex: 0.0;
  } @else {
    -webkit-box-flex: 1+$grow/10;
  }
  -moz-flex-grow: $grow;
  -webkit-flex-grow: $grow;
  flex-grow: $grow;
 }

 @mixin flex-shrink ($grow) {
  @if $grow == 0 {
    -webkit-box-flex: 0.0;
  } @else {
    -webkit-box-flex: 1+$grow/10;
  }
  -moz-flex-shrink: $grow;
  -webkit-flex-shrink: $grow;
  flex-shrink: $grow;
 }

 /**
 * 子元素的显示次序 item
 * @params {integer}
 */
 @mixin flex-order ($order) {
  -webkit-box-ordinal-group: $order;
  -moz-order: $order;
  -webkit-order: $order;
  order: $order;
 }

请问 -webkit-box-flex: 1+$grow/10; 这里计算的依据是什么呀