You’re missing my point here.
You wrote:
However, in your template you have:
This means that your template is set up to handle multiple results. It’s designed to render a queryset and not an individual entry.
(This issue has nothing to do with your calculations - this is a separate point addressing your understanding of the model and the data you are working with.)
Now, regarding your calculation, you could create another method on the model to perform the complete calculation, and reference that in your template.