How is SLA report calculated (specifically internal service check related)

Could somebody explain how is SLA calculated for internal service checks?  I read this but it didn't seems to apply to the report I created.

/topic/1015-a-closer-look-at-how-sla-is-calculated-in-reports/

I have an internal service check scheduled to do stat test every minute. There were 4 network errors (status = 8) within the last 24 hours. I created a report using SLA widget to report 24 hours SLA and it showed 100% during the past 24 hours. If it's calculated to the seconds, the calculation would be: (24(hr) x 60(min) x 60 (sec) -4 (errors))/(24(hr) x 60(min) x 60 (sec)) x 100% = 100.00%, which seems to match. So, to confirm, I changed the report config to narrow down to 2 hours (in which 2 of the errors appeared), which should show SLA of 99.97% (i.e. (2x60x60-42)/(2x60x60)) but SLA report shows 100% still.

Thanks,

Mary