Changing PCB trace pitch as a way to controll sdd21

[yefj]

Hello , I tried to build a differential line and controll the SDD21 as shown in the photo.
SDD21=S21-S41

i tried to increase the absolute s41 but the sdd21 is the difference between two complex and as you can see in the photo.
i cant get the sdd21 smaller.
Where did i go wrong?
Thanks.