Thank you for your feedback. Currently, you cannot set JustCode to analyze only the open files. However, there is a functionality in JustCode that will let you ignore errors/warnings coming from specific projects. You can add the project name to the "Exclude project filters" and JustCode will not report any errors from it. You can find this filter in JustCode->Options->Code Analysis".
If you have any more questions or remarks, please do not hesitate to contact us again!
the Telerik team