Confusion over "Operational Hours" and "site" time settings

Confusion over "Operational Hours" and "site" time settings

Hi,
 
Our organisation consists of three regions - UK, US & HK.
 
We have a single IT support team based in the UK that provide technical support to all three sites.  Our operational hours are 08:00 - 19:00 GMT and anything else is considered "out of hours" support.
 
What is the best combination of settings to use to ensure that the SLT calculations are all based on OUR operational hours rather than local time for each site?
 
We currently have the local times set correctly for each region and "refer default settings" enabled.  However, we often breach SLTs as the calculations appear to be based on the local time of each site rather than GMT.  The only current workaround we can find is to use GMT for all sites which is clearly undesirable.
 
Thanks.

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