My name is Allyson, to give you an answer on new homes it all depends on how much they want to move the units. I've made some ridiculous offers for my clients and helped them negotiate a great deal. Remember you need to bring your agent with you on your first sign up visit, otherwise they won't let them help you negotiate the deal.
Also remember that with new home builders they won't be insulted by a low ball offer and there are other considerations besides price, you might want to consider more upgrades that can also be negotiated.
What I would do if i was your realtor is pull up the comps, what the other units have been going for then talk to the agent and find out how badly do they want this moved and then begin looking at what they will throw in (upgrades) then you can really get a better idea of what to offer. (your percentages are not to far off at all)
If you would like some assistance email me at email@example.com or call me at 408-705-6578 you can also go to my web site and check me out there at http://www.homeasbyallyson.com.