The Laura Mercier Pressed Powder will not give you the coverage of a foundation. It is more in line with a setting powder.
If you want to use the powder as your foundation, then you should get either the Bare Escentuals Mineral Foundation or Laura Mercier Mineral Powder Foundation.
Personally, I would recommend the Laura Mercier Mineral Powder Foundation because I think it has a smoother finish than the BE powder and it gives better, more even coverage, too. You can find it at: http://sephora.com/browse/product.jhtml?id=P181802
I hope this helps you.