nope, need good old devision

events.ics

@@ -11,7 +11,7 @@ UID:{{ event.uid }}
 ORGANIZER;CN="Chaotikum e.V.":MAILTO:{{event.contact}}
 LOCATION:Chaotikum
 {% capture textSize %}{{ event.short | size }}{% endcapture %}
-{% assign rounds = textSize | modulo: 70 %}
+{% assign rounds = textSize | divided_by: 70 | plus 1 %}
 SUMMARY:{{ rounds }}:{{ event.short }}
 CLASS:PUBLIC
 DTSTART:{{ event.eventdate | date: "%Y%m%d" }}T{{ event.eventdate | date: "%H%M%S" }}Z