inner-outer-radii.less

@baseFont: 16;
@outer: 50;
@margin: 20;

// Mixins
.inner-box() {
	@inner: @outer - @margin;
	@innerRem: @inner / @baseFont;
	@MarginRem: @margin / @baseFont;
	border-radius: ~"@{inner}px";
	border-radius: ~"@{innerRem}rem";
	margin: 0  ~"@{margin}px";
	margin: 0  ~"@{MarginRem}rem";
  padding: 0  ~"@{margin}px";
	padding: 0  ~"@{MarginRem}rem";
}
.outer-box() {
	@outerRem: @outer / @baseFont;
  @padding: @margin;
	@paddingRem: @margin / @baseFont;
	border-radius: ~"@{outer}px";
	border-radius: ~"@{outerRem}rem";
	padding: ~"@{padding}px" 0;
	padding: ~"@{paddingRem}rem" 0;
}

// CSS
.outer {
	.outer-box();
}
.inner {
	.inner-box();
}

page.html

<section class="outer container">
  <aside class="inner container">
    <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Vivamus at odio faucibus, venenatis quam vel, dapibus massa. Quisque bibendum in ante in ultrices. Phasellus viverra egestas sapien, in euismod elit auctor eget. Vivamus vitae justo tellus. Morbi fermentum pretium tortor non euismod.</p>  
  </aside>
</section>