How low is TOO low for an initial lot offer?

[Mitch Bernstein]

I'm looking at purchasing a lot to build on for personal residence.   For obvious reasons I want to get the lot for as little as possible.  Getting it for the price I want is where I need some guidance.   Should I offer %50 below asking, 60%, etc?

Last time a made a low offer on a different property the owner was so insulted they didn't even negotiate.  

This property has some unique features (good schools, low taxes, etc) so I'd like to negotiate and not risk having them scoff and slam the door so to speak. 

 The lot is wooded and undeveloped, and is owned by the neighbors who we believe are going through a divorce and offloading this as they are moving out. 

Any thoughts?