CST waveguide port- a reasonable way to feed asymmetric microstrip

Hi, I am hoping to use CST to get the impedance at one end of an asymmetric microstrip as accurate as possible.
The cross section of my microstrip is as shown in attachment.
I am really confused what dimension of waveguide port should be to get an objective and accurate line impedance there.
Should it still be symmetric, can it go beyond the edge of microstrip?
If confined to microstrip, the left part of waveguide is only 1.5times the width of top strip.
Thanks so much for any idea and suggestion!