You shoud pass 2 arguments, the min and de max chars. The template tag will truncate the text on the last space
{% load smarttruncate %}
<div>
My description is {{ description|smartcut:"50:70" }}
</div
This is specially usefull when you don't have controll over the meta titles,descriptions...etc. And you can still respect the max lenght.
<meta property="og:title" content="{{ description|smartcut:"50:60" }}">
<meta property="og:description" content="{{ meta.description|smartcut:"120:158" }}">