Yes, agree with you advise. But i do have another concern that in the situation if the business unable to provide the half hourly volume, i can only use the manual calculation method that include shrinkages and OCC.
And if i juz average the volume to 48 interval since 24 hours, the FTE require will not be accurate due to call arrival pattern as you advise.
Threfore, i would like to find out how to make the manual formula calculation result get closer with Erlang C ( with SLA consideration ). Thus, i had make some research and trial which giving me if i want 90/20 will need to add in additional 67% FTE in the manual calculation or 80/20 will be addition 51%. This result are finding using Erlang C calculation with weekly volume by interval volume and AHT compare with manual calculation juz use above method.