Age dating argon
Now the bad news is that there is no way we can somehow manipulate this data to give us a correct date for the sample.
A crucial point to note is that because K are isotopes of the same element, they have the same chemical properties.
J is not calculated on theoretical grounds, but is found experimentally; alongside the sample we're interested in, we irradiate and then heat a sample of known age (a standard).
Measuring the Ar emitted from the standard, and knowing the time t that it was formed, we can put these figures into the equation above and solve it for J.
So now we know J, and we have measured the R-value of the sample we're actually interested in dating, so we can use these data to solve the equation for t, giving us the age we're looking for.
You will note that this means that we have to be able to date some rocks accurately using some method other than Ar-Ar, so that we can find a standard to use for the determination of J; fortunately we can do this, and geologists have put a lot of effort into identifying rocks which can be accurately dated and used as standards.