The accepted method is to total all the calls that were answered within the SLA target and divide this by the total number of calls answered to produce the overall SLA percentage. Some switches will report this figure directly, e.g. the Lucent Definity defines the variable ACCEPTABLE in the VDN and Skill tables to count the number of calls answered within SLA. Otherwise multiply the number of calls answered in a queue by the SLA percentage for that queue to get the acceptable calls figure, add these together and divide by the total answered.
This method is also used to calculate the monthly average from the daily figures for that month.
(Note that some switches report the SLA as a percentage of offered calls rather than answered calls in which case use the offered calls figure instead)