Your question is actually quite common. How to deal with resources in evaluating projects in a portfolio (or projects in a company with a single portfolio) when I don't really capture detailed data.
When you think about this, the questions you want answered are:
- What are my people doing right now?
- What will my people be doing x number of months from now?
- How will adding this project affect my people's ability to do the existing work?
Looking at personnel, you want to find out what projects you have, who's working on them, when those projects end, and how much time it will take to complete. From this, you can create an Excel spreadsheet or MSProject chart to map your resources. For this, rough estimates are fine. You can estimate full-time allocation on a week-by-week basis for your people.
From this estimate, you can then see how a new project will affect your workload. You can see who is available when the project starts, who is available during the project, and who will become available when the project ends.
Let me know if you have any other questions or thoughts.