An SLA of "99.9%" sounds like a rounding error until you convert it into hours offline and money lost. Enter what your site earns and the uptime you are being promised.
This is arithmetic on the numbers you enter, not a prediction. Downtime rarely falls evenly, and an outage during your busiest hour costs far more than the average. Treat the result as the floor, not the ceiling. Most SLAs also refund only a percentage of your hosting fee, never your lost revenue.