I haven't done any RFP responses for a while but this question always used to make me furious. Our competitors (some of who had had major incidents in the past) claimed 99.99% availability or more, knowing they would never have to prove it, and knowing they were actually 100% until the day they weren't.
We were more honest, and it probably cost us at least once in not getting business.
An SLA is a commitment, and an RFP is a business document, not a technical one. As an MSP, you don’t think in terms of “what’s our performance”, you think of “what’s the business value”.
If you as a customer ask for 5 9s per month, with service credit of 10% of at-risk fees for missing on a deal where my GM is 30%, I can just amortise that cost and bake it into my fee.
We were more honest, and it probably cost us at least once in not getting business.