Quick Question: Short Sale Offer - Preapproved Number

[Chris S.]

I'm going to look at a short sale.  They gave a preapproved number.  It's a pretty good deal as is.  Should I offer 10 - 15% lower than asking?  It's been around for a while and I don't think there are any other interested parties. 

Does this add a lot more time to the process?

I'm not really experienced with short sales, so any advice with anything on a short sale would be awesome.  Thanks!