Sum of one checkbox field
What you could try is putting the "if statement" as a condition in the for loop like this:

{% for premiereItem in entry.kompositonsart if premiereItem.value == 'ua' %} //the checkbox loop starts
    // only return checked value with 'ua'
    {{ countPremiere }}
{% endfor %}


I'm not really sure what you are asking but if my thoughts are right this should do the trick. This way it wont effect any other regular loop functions like a loop.index if you are going to use it.