A bag that's too warm, you can unzip. One that's too cold, you are cold.
Generally a good bag (850+) down rated to 0 C should cover a lot.of conditions. And if its colder then you can wear a baselayes and put your insulated jacket over the bag to help keep warm.
What i am comfortable with isn't any help to you. We all sleep different.
https://www.andy-kirkpatrick.com/articles/view/sleep_tight
https://www.phdesigns.co.uk/temperature-ratings-for-down-sleeping-bags-and-down-clothing