SLA Calculation including weekends

apologies, I misread the requirement -- I've revised the approach / formula per below -- and reattached for sake of download / translation

Formula:

C2: =SUM(INDEX(NETWORKDAYS.INTL(A2,B2,{"0000011","1111101"},_holidays)*{12,9}/24,0))-IF(NETWORKDAYS.INTL(A2,A2,"0000001",_holidays)=0,0,MEDIAN(MOD(A2,1),INDEX(CHOOSE(MATCH(WEEKDAY(A2,2),{1,6}),INDEX({"08:00","20:00"},0),INDEX({"09:00","18:00"},0))+0,0))-LOOKUP(WEEKDAY(A2,2),{1,6},{"08:00","09:00"}))-IF(NETWORKDAYS.INTL(B2,B2,"0000001",_holidays)=0,0,LOOKUP(WEEKDAY(B2,2),{1,6},{"20:00","18:00"})-MEDIAN(MOD(B2,1),INDEX(CHOOSE(MATCH(WEEKDAY(B2,2),{1,6}),INDEX({"08:00","20:00"},0),INDEX({"09:00","18:00"},0))+0,0)))

copied down

as before there will be a better approach, but hopefully this helps you a little?