Auto levelling unassigned tasks - way to limit to 100% units?

Hi. I am using OmniPlan 4 Pro under macOS and have a number of non-dependant tasks but cannot work out how to limit how many can be automatically scheduled in parallel via levelling without assigning all unassigned tasks to a fake “Unassigned” user. Is there a way to limit unassigned tasks to say that of a single resource without actually creating and managing such a resource? I find it handy to have the project auto-level and show a worst-case end date based on one resource. As tasks gain an actual resource, the plan would not change but if a second was added, the projects end date would come back a bit based on effort and duration, etc. Thank you, Jasper.