...
.span5 {
  width: 5%;
}

.span4 {
  width: 4%;
}

.span3 {
  width: 3%;
}

.span2 {
  width: 2%;
}

.span1 {
  width: 1%;
}

@iterations: 5;
.span-loop (@i) when (@i > 0) {
    .span-@{i} {
        width: ~"@{i}%";
    }
    .span-loop(@i - 1);
}
.span-loop (@iterations);

.span-5 {
  width: 5%;
}
.span-4 {
  width: 4%;
}
.span-3 {
  width: 3%;
}
.span-2 {
  width: 2%;
}
.span-1 {
  width: 1%;
}

.custom-loop( @base-value:@n ; @n ; @unit : px; @j : 1 ;@_s:0 ; @step-size:1  ; @selector:~".span-"; @property : width)
when not(@n=0)  {

  @size : @base-value+@_s;
  @{selector}@{j}{
    @{property} : ~"@{size}@{unit}";
  }
  .custom-loop(@base-value ,  (@n - 1), @unit , (@j + 1) ,  (@_s+@step-size) , @step-size,  @selector, @property);
}

.custom-loop(@n:3);

.span-1 {
  width: 3px;
}
.span-2 {
  width: 4px;
}
.span-3 {
  width: 5px;
}

.custom-loop( @n: 3 , @base-value:1, @unit: '%', @property:font-size, @selector: ~".fs-", @step-size: 2);

.fs-1 {
  font-size: 1%;
}
.fs-2 {
  font-size: 3%;
}
.fs-3 {
  font-size: 5%;
}

@iterations: 100;

// helper class, will never show up in resulting css
// will be called as long the index is above 0
.loopingClass (@index) when (@index > 0) {

    // create the actual css selector, example will result in
    // .myclass_30, .myclass_28, .... , .myclass_1
    (~".span@{index}") {
        // your resulting css
        width: percentage((@index - 1) *0.01);
    }

    // next iteration
    .loopingClass(@index - 1);
}

// end the loop when index is 0
.loopingClass (0) {}

// "call" the loopingClass the first time with highest value
.loopingClass (@iterations);

.loopingClass (@index) when (@index > 0){
  .span_@{index}{
    width: @index px;
  }
  .loopingClass(@index - 1);
}
.loopingClass(5);

.span_100 {
  width: 100 px;
}
.span_99 {
  width: 99 px;
}
.span_98 {
  width: 98 px;
}
.span_97 {
  width: 97 px;
}
.span_96 {
  width: 96 px;
}
.span_95 {
  width: 95 px;
}

and e.t.c.