Pricing on Used Boats from Dealers

Hey guys, I tried searching the forums but to no avail, if this question has been asked a bunch already sorry.

I've seen that you should get out of dealership, with some haggling, with about 20% off a sticker price with a new boat. Is there a similar rule of thumb for Used boats? Or is NADA your best bet and just use that to help with haggling.

I'm looking at a 2011 242 Supra that's listed at 57k, with about 70hrs. I'm would feel really comfortable if I got out of there under 50 Any opinions?

Also if a boat catches your eye up in your area that are in a similar price range let me know... I'm gonna be wakeboarding 99.5% of the time and right in that intermediate level. Thanks!