Custom formula using Workflow rules?

I have Total capacity available of all resources / week. We have a custom field called Estimated dev hours at the project level.

I need to calculate the 'remaining total capacity' as and when resources are added to projects.

Example:

5 resources available for 30 hours/week

Total available capacity right now = 150 hours/week

5 projects have been entered

When I assign Resource 1 to Project 1 for 10 hours (this is the estimated dev hours), I want Remaining Capacity to show 140 hours/week. (Total capacity - Estimated Dev hours/ project)

This will help me determine how I can distribute remaining projects among other resources based on availability. As and when a resource is added to a project, I want the remaining capacity to update accordingly.

Challenge I am facing is - Capacity is at the task level while Estimated Dev hours is at the project level.

Using custom formula at a report level is not possible (I haven't figured out a way to do that). I'm wondering if I can use workflow rules to run this calculation somehow? If yes, I'd appreciate guidance on how to achieve this.