Nathan, I think you followed it pretty well. To get the X face value, I am basing it on what a $1 of silver (90%) weighs.
The act establishing the silver dollar (Coinage Act of 1840, I think) established the content of a "dollar" to be about .72 ounces of silver.
A dime would have 1/10 of that, a quarter 1/4 of that, half dollars 1/2 of that, etc. so as far as silver content, the denomination doesn't matter. No matter what combination of coins you have, if it's a dollar face value, it will be about .72 ounces of silver.
If you look in any Red Book, under dime, qtr., half, etc. it will give the silver content of the denomination. For 40% halves, take the silver weight of it and multiply it X 2 to get the $1 value. Each one is about .14 thus the .28 figure.
Remember also that the phrase "one thin dime" has meaning as well. If it is worn down, then the weight is off.