I don't know the recipe you're referring to but the general rule of thumb is no less than 2 tbs sugar per egg white. If there is cream of tartar in the recipe you'll only need the sugar (there should be 1/8 tsp per egg white). You can also sub powdered sugar for the granulated (if there is no cream of tartar in the recipe) as the cornstarch will stabilize the meringue.
But the nature of the meringue will attract moisture with the amount of sugar. Be sure to put on the meringue when the filling is piping hot...meaning to make your meringue before the filling. That way the bottom of the meringue will be "cooked" by the residual heat.
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