Lead core depth has many variable that determine how deep the lure will be at; speed, water density which is effected by temperature, diameter of the lead core and leaders, under water currents, drag constant of the lead core, leader material and lure.
A rule of thumb is 5 ft per color at 2 mph for lb test of 18 and greater and then add or subtract a little depending on speed. The best advice is to listen to the VHF and use what other guys are using to catch fish, speed and number of colors in the water.

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Gus

Another variable may be the kind of lead core line you have. I have the "no lead" lead core and I've noticed it does not go down as fast as the regular lead core.

Louis

Buy the Chappy's guide in the 2 inch size. All colors have been working for me in any weather... the gold/red is my favorite

The thermocline is starting to show up. Generally speaking leadcore sinks 4-5 feet per color. Like others have said it varies depending on a lot of factors. At the speeds you say you are trolling however I would say that your lead is sinking 4-5 feet per color. Right now with the thermocline showing up I would say you want to be fishing 7-10 colors out or set riggers between 32 and 42 feet.

Thanks guys. I'm heading to Alton bay in the a.m. I've also ordered my chappy lures.