forms – Indicate a Input Value is the Primary Unit

I’m currently putting together a UI content block that would allow a user to select a unit, unit size, position and input the needed crew values. The end user should be able to determine whether or not the initial squadron size is the primary value in this simple equation:

Sqd 15 – Sqd10 = Difference of 5

Not entirely married to the current idea of the Primary Value checkbox after the Sqd size number input and was wondering if someone out there ma have a better approach.