You can use a conversion equation to get you started but you will have to do the calculations based on a number a variables. What you really need to match is fermentables. The ppg (points per gallon/gravity points contributed by each pound of product to each gallon of wort) will vary for different brands of extract and for different types of malts. Your mash effeciency or the actual ppg you realize from each pound of malt also needs to be factored.