simple productivity measue

[etaf]

I'm looking for a very simple productivity measure to compare week previous year on current week.

Calls/FTE is the ratio I'm looking at - but also need to weight it depending on the SLA performance from target of 80% variance can be + 19% or - 25%

I'm looking to simplify some highly complex call centre stats and try and show a financial benefit - saving through increased productivity

Ideally I'm looking at simply

FTE - Full Time Equivalent (People)
Calls Raised
Calls Deployed
Service Level Agreement SLA

Then using a ratio of call/person

I would like to use
Calls Deployed / FTE
Simply that and show an improvement following intervention
However, they also have an SLA to meet and sometimes they exceed - because too many people in and sometimes they miss the sla

so
call deployed / FTE / SLA

anyone use a very simple callcentre statistic that works out the financial saving

The operation has some complex call handling software and plans weekly - so loads and loads of data around for all sorts of variables, and I have built a very complex model

BUT need to simplify

Any help appreciated