We have the same issue. We forcast on 80/20 SLA because that seems to be the only way Excel/Erlang works but the Contact Centre is targetted on 5% abandon rate. This should mean, assuming that you are using excel and the erlang add in, that results are better than forecast because of the imperfactions of excel and will therefore deliver a better level of service than you hope?!
There appears to be, after looking at for a few years, no direct corrolation between SLA and capture rate other than if you get a good SLA you will by default get a good capture rate.
We have looked to try to predict what either the % in 20 secs would be or the number of seconds that the 80% would be answered in but there are too many potential options to get it to work properly. It can also depend on the type of call, ie does the customer really want something you are selling/providing and are they prepared to hang on etc and your customer base.
I have taken to try to get the Contact Centre to report on SLA rather than capture/abandon rate as this, in my opinion, gives a better picture of what level of service is being provided to customers.
If you ever find the result, please let me know!