This requires two things
-
Running simulation sweep of radial length vs Z (R+jX)
-
Making a prototype, trim radials, and measure Z (R+jX) for each length of radial to determin optimum length of radials.
Couple of years ago (March 2016), I have run simulation of a similar (but not identixal) design to find optimum radial length. Due to differebce in design, and using system impedance of 75 ohm instead of 50 ohm, It will not fit to V-stub, but will give an idea that reducing the length of radials improves performance.
https://discussions.flightaware.com/t/why-are-all-wireantennas-differnt/18160/9