I am facing a similar situation in which I have a 'dummy project' and top level for a department. A resource can or cannot have a work task assigned, so primarily should be 0% allocated to this project. The hurdle is that if I confirm the  100% allocation on other projects and a short task (lets say for few hours) at dept level come up, I cannot readjust all project allocations automatically. Is there a way to do it?

I primarily was going through following question:
'

Resource allocation when adding a team member is not zero?!'

not sure how the site works if I have to ask a linked question. Hope I have clarified my query; if not please feel free to ask.