Typically, 1 pound of LME will yield a gravity of 1.030 in gallon of water. So assuming that you are doing a 5 gallon batch, you will lose 1.006 off of your OG. This sounds right to me, but someone who is a little bit more awake may be able to do better math.
If you can I would recommend just using some light or extra light DME if you have it. Generally 1 lb grain = .75 lb LME = .6 lb DME
As for the powder, or flour in your specialty grains, don't sweat it. It will not do any harm to your beer. Most of it will either fall out in the trub when you boil or during fermentation.