Hey Henri,
Here you go:

# common/templatetags/mathtags.py
from django import template
register = template.Library()
@register.filter()
def divide(n1, n2):
    try:
        return n1 / n2
    except (ZeroDivisionError, TypeError):
        return None

@register.filter()
def floor_divide(n1, n2):
    try:
        return n1 // n2
    except (ZeroDivisionError, TypeError):
        return None

@register.filter()
def percentof(amount, total):
    try:
        return '{:.1f}%'.format(amount / total * 100)
    except ZeroDivisionError:
        return None

@register.filter()
def multiply(n1, n2):
    try:
        return n1 * n2
    except TypeError:
        return None