hmmm.. i would actually ask the folks at the homebrew place HOW fermentable their malto-dextrin is. I have seen it quoted from different sources as low as 4% and as high as 20%.
The higher the fermentability, the more careful you need to be. You might have to cut down on the priming sugar when you bottle, or you could get some bottles exploding.
Whatever their quoted % is for the fermentability, multiple that by 8 oz (the amount you are adding.) Whatever number this produces is how much less corn sugar you should add when priming the beer for bottling.
Austin Homebrew says their MD is 12% fermentable. That means your 8 oz of MD contains (0.12 * 8oz =) 0.96 oz of fermentable sugar.
So, leave out one ounce of corn sugar when you bottle to be safe.