Are you sure it's off?
Here is an easy, very accurate way to test: $20 of dimes or quarters (or mixed) equals exactly 1 pound. (50 cent pieces or Eisenhower dollars also work, but who sees those?) No pennies or nickels.
So 5 quarters = 1 oz, 40 = 8 oz, etc.
Any mix of these coins works -- $2.50 worth equals 2 ounces, $5.00 worth equals 4 ounces, etc.
I'll vouch for puzzler's method. I had to calibrate some scales for work using a standard weight. Just for kicks I compared to a stack of quarters and found that 5 quarters do indeed weigh 1.00 ounce. Didn't know dimes followed the same rule, but I don't weigh my spare change that often.