I work on a web application to file a specific kind of county taxes. Our company wants our state to mandate that counties must accept electronic filings (as opposed to paper) from any system that meets some sensible requirements for uptime, security, data validation, etc. (Yes, this would help us as a business, but it would also force county governments to be more efficient.)

We're creating a draft of those requirements to be reviewed and tweaked with the state. One of the sections is "availability." We want to specify something reasonably high, but not so high that any unexpected problem will get us (or a competitor) penalized.

How do we decide what's reasonable for availability requirements?