@defaultfontsize: 16;
@defaultlineheight: @defaultfontsize*1.375;

.font(@fontsize:16,@lineheight:22,@multiplier:1,@lineheightreducer:1) {
	font-size: (@fontsize/@defaultfontsize)*1em;
	line-height: @lineheight/@fontsize/@lineheightreducer;
	margin: 0 0 (@lineheight/@fontsize*(@multiplier/(@lineheight/@defaultlineheight)))*1em 0;
}

.boxes(@paddingtop:1,@paddingbottom:1,@bordertop:0,@borderbottom:0) {
	@border-top-width: @bordertop*1px;
	@border-bottom-width: @borderbottom*1px;
	padding-top: e(%("%sem%s", (((@paddingtop*@defaultlineheight)-@bordertop)/@defaultfontsize), `@{bordertop}>=1?'; border-top-width: @{border-top-width}':''` ));	
	padding-bottom: e(%("%sem%s", (((@paddingbottom*@defaultlineheight)-@borderbottom)/@defaultfontsize)*1, `@{borderbottom}>=1?'; border-bottom-width: @{border-bottom-width}':''` ));
}

h1 {
 .font(48,25);
}

Simple snippet for calculation font-size, line-height, and margin-bottom.

Line-height calculator LESS

CSS