As long as the solution is pure, your idea of making standardized $\ce{NH4NO3}$ solutions and comparing conductance should work.

Caveat: This works as long as conductivity vs. concentration is a monotonic function. For some other substances is not, e.g., for $\ce{H2SO4}$, which, at STP, has a distinct peak in conductance at ~30%, and then drops off, so that there are two points on the curve at which conductivity is the same, but concentration quite different.

This is also the case for $\ce{NH4NO3}$ above 8.0 m/mol kg^-1, but, in the region in which you are interested, it should not be an issue.

However, there is another, simpler, rather old alternative: use a hydrometer. The density of $\ce{NH4NO3}$ is a monotonic function. And others have already done the work for you, charting density against concentration.

_Eureka!

As long as the solution is pure, your idea of making standardized $\ce{NH4NO3}$ solutions and comparing conductance should work.

Caveat: This works as long as conductivity vs. concentration is a monotonic function. For some other substances is not, e.g., for $\ce{H2SO4}$, which, at STP, has a distinct peak in conductance at ~30%, and then drops off, so that there are two points on the curve at which conductivity is the same, but concentration quite different.

This is also the case for $\ce{NH4NO3}$ above 8.0 m/mol kg^-1, but, in the region in which you are interested, it should not be an issue.

However, there is another, simpler, rather old alternative: use a hydrometer. The density of $\ce{NH4NO3}$ is a monotonic function. And others have already done the work for you, charting density against concentration.

_Eureka!