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.
Yeah that's what I am learning. It's a lot of money if you end up with the wrong bag though.
Thanks for links!