I was looking at my Bingham's catalog and they have a draw weight chart based on the thickness of the lamination stack. It appears on their design recurve reducing the limb thickness by .006" will yield a 5lb decrease in draw weight, .005" is 5lb on the longbows, so I'm guessing this will give you a ballpark on how much you can take off to get the poundage down to where you want it. You could also trap the limbs (lots of info on the search) to reduce the poundage by a couple # also.
I'm sure someone on here with more knowledge might know some other ways. Whatever you do go slow, measure often, and make sure you take even amounts off both surfaces so you don't change your tiller.