Ok, How about a slightly different approach perhaps?

Instead of applying a percentage value directly to an inline `style` attribute in the markup, place it in a data attribute:

    <span class="progressspan" data-percent="{{ poll|get_users_voted/poll|get_voters_per_group*100 }}"></span>

Then in the JS, get this value using:
    var $this = $(this);
    var percent = $this.data('percent');

Then it's much easier to limit to a max value of 100%, after which you can apply it:

    $this.filter('span').css('width', percent + '%');

This feels a bit cleaner maybe?