Is this really standard practice for measuring the SLA? The contracts I've seen for a couple small businesses are generally per billable period. Which always m

Is this really standard practice for measuring the SLA? The contracts I've seen for a couple small businesses are generally per billable period. Which always made sense to me. I pay you for 99.5% uptime this month. If you don't achieve it, then I get a discount, as simple as that. If your availability is below that, I don't pay full price for the billable period and then reconcile at the end of the year. Any links or general advice on this topic anyone has I'd be pretty interested on finding out if there's a general consensus of it being done differently?