I usually figure on slack water about two and a half hours after high tide for FI inlet. Not always a perfect method but it's pretty close. Some other publications give you current tables as well as tide charts. Currents are what you want to look for.
Just note when the current changes sometime when you're out there, come back and check the tide charts for that time. Do this for different points along the inlet. The tide at the coast guard station is twenty minutes behind the inlet. And Ocean beach can be two hours behind the inlet. Use this information and you can really increase the amount of prime ebb tide time you get to fish.
Also,currents follow the tides differently all around the island. At the Stony Brook Harbor inlet slack water comes about 45 minutes after high tide.