You live in Portland...go to a Market of Choice or any fancy schmancy grocery store, go to the baking section and look for rice syrup...shouldn't be a problem to find it.
If you can really only find powder (or don't want to order the syrup online if you can't find it in a store) I would go with the LME/DME concentration ratio...DME being 20% more concentrated weight-wise than LME. Or you can calculate how many points the rice syrup is supposed to get you based on the recipe using software (beersmith has about 1.032 PPG) then mix up a gallon of water and enough rice extract powder until you hit that same gravity, then add it to the kettle.
Hope this helps.
Fermenting: Flanders Red Ale
Conditioning: Blackberry Mead, Altbier, Cardamom Saison
Kegged: Vanilla Bourbon Porter, Belgian Strong Dark