what is a acceptable of percentage of available time in productive time.

The calculation that I am using for Productive Time is =

Sum (Available Time + Talk Time + ACW Time) / sum (Occupied Time = Staff Time - Lunch and Paid Break.)

Contact centre is
36 FTE
open 24hrs
AHT 6.5
26000 calls per month

Productive target being 87% and Occupied time shrinkage of 13%.

Bit of a tricky one.

What is your service level target?

If you are using erlang in your staffing calculations it will calculate more staff than you actually need and this will get worse with a higher service target. Combine this with low contact volumes and you can be planning for c40% wait time, further combined with 24/7 and I wouldn't be surprided if you are close to 50/60% wait.

Service Level Target 80/20

Also using erlang C calculation.

Jan 08
53% available of productive time, 84% productive time.
Target shrinkage of 13%
unproductive of 3%

What you have quoted seems about right. As an alternative you may want to look at historical staffing and performance and use this as a guide to trimming your requirement and ergo decreasing the wait time. Always difficult though with low volumes and long operational hours.