One ounce of 90% silver quarters is $1.25 face value. So 4 quarters is 80% of one ounce. But the silver content is a mere 90% which works out to one dollar face value having 72 % of an ounce of silver.
$1,000 face value of 90% junk silver is considered to be 715 troy oz. Perhaps uncirculated 90% silver would be 720 troy oz. I'm not certain of that but that might make sense because junk 90% silver has some wear and therefore would weigh a little less than uncirculated 90% silver.