Ah, in that case Michael you are double-counting the avail/idle time.
The Agent formula dictates what the avail/idle time should be. In your one of your earlier replies you show that the agents required in any one hour is 4
The Agents formula though does not consider that the agents will be busy 100% of their time. If we consider that in this hour the agents have to handle 2500 / 168 calls = 15 calls (approx) and that each call takes 500 seconds then we can estimate how much work has to be done in that hour
15*500 = 7,500 seconds of work to be done, that equates to 2.1 hours (7500 / 3600)
The Agents formula tells us that we need 4 agents in that hour all supplying an hour each to do 2.1 hours of work, therefore the occupancy is 52%.
In your overall calculation you inflate the HC requirement by saying that 25% of 75% of the 40 hours that an agent works is lost to availability, therefore you'd need to remove the assumption on occupancy.
As Dave mentioned earlier it would be much more accurate for you to forecast each hour individually to come up with the HC requirements. In doing this you can apply the Agents formula separately to each hour. This means that the formula can take into account when the operation can perform at different occupancy levels.
2500/168 = an average of 15 calls per hour
Agents(80%,30,15,500)= 4 agents, 2.1 hours of work required at a 52% occupancy
if we double the number of calls and get the Agents formula to work out the requirements we find that
Agents(80%,30,30,500)= 7 agents, 4.2 hours of work required at now a 60% occupancy level.
This shows that occupancy varies when the call total changes - greater calls, greater occupancy. It appears that you have noticed this yourself when planning for larger operations as you used a higher occupancy assumption (85%).
I'd advise that you use a hourly call forecast, apply the Agents formula to each hour, sum those totals for the week and uplift the number to take shrinkage into account and then divide by 40 to get you HC total.
Hope this helps.