What elements of the budget will be flexible/inflexible?

This varies for each solicitation. The Business Office will review the solicitation document for any required budgetary items and let the PI know. Including a reasonable amount of effort for the PI is one of the items that you need to include. Determining what reasonable means is highly dependent on the proposed scope of work. As PI, you need to be able to justify that you can complete your duties as PI of the project in the time you've requested in the budget. The Business Office advises that you do not go below 0.5 month of effort for a year, though there are certainly exceptions.