First, 1g/lbs protein is broscience.
You're right, 0.75g/lbs is suffice.
In fact, overkill of protein increases IGF-1 levels and increases your risk of cancer.http://www.cell.com/cell-metabolism/abstract/S1550-4131(14)00062-X
Now, the whole "usable protein" thing only applies if your diet is extremely limited,
meaning you're getting your calories ONLY from peanut butter, or beans, etc.
If you're eating a diet with variety, it's fine. One example is beans and rice.
Beans have a high amount of certain types of amino acids, but extremely low in other types of amino acids.
Rice makes up for the low amino acids in beans.
Here's a PDF that shows you what the limiting amino acids are in certain foods, and what to pair them with:http://www.savvyvegetarian.com/articles/plant-food-protein-chart.pdf
By the way, you DON'T have to eat the complimentary foods together in the same meal.