Definitely as you mention you should use the affirmative language.
To begin, the affirmative language is the convention and there's not any real benefit in changing it for the past form. This the first big reason.
The state in this control is dictated by the checkbox itself so you don't need to reflect the state in the text.
Classic examples: "I accept the terms and conditions", "Include extended 2-years warranty" etc.
Something like "I accepted the terms and conditions" may bring doubts like "Ok, so you accepted this a second ago or one month ago?" I mean confirming that you did some action in the past but maybe you wouldn't do it now.
You could say, okay but you are cherry picking one case and could work with the rest, but as a general design principle it's recommendable to maintain consistency between controls, so they aim a specific purpose across your application, even more if they're part of a convention.
Other problem may be accessibility. I'm not really into this topic, but I can imagine a screen reader who speaks "checkbox" "Included in calculations" and I doesn't sound like and option but just a declaration that will bring at least some confusion.