Impedance control for small antenna feed?
You have to use the wavelength in the medium where the wave is traveling. On PCB's, waves travel more slowly than they do in free space. For an internal trace (stripline transmission line) the speed is around 47% of free space, so the wavelength is around 47% also (waves bunch up closer together when they slow down). So rather than 122mm, the wavelength will be around 57mm. This puts you a bit over the maximum length (which would be 5.7mm according to the rule of thumb).

However, you are probably not using an internal trace. For an external trace (microstrip) the propagation speed is slightly faster (around 56% of C) but the result is roughly the same.

So I recommend that you simply design your feed trace to be around 50 Ohms. This is not difficult. You can find online tools to help you.