Ripper,
Based on my experience launching a 14 foot Lund at Ambleside, I think you would want at least and 8 to 10 foot tidal height. For me with a light boat, its no probs at all. But with a bigger boat and deeper draft, your truck would end up in the sand beyond the reach of the concrete ramp at lower tides. Could get stuck and boat could bottom out.
Just my $0.02.