Calculation of microstrip impedance in topology extraction (Allegro PCB SI)

I am new to PCB SI. I have got a problem of calculated microstrip impedance in Allegro PCB SI.

I could address the problem that the doubtful calculation results were the microstrips whose traces were tapered trace and manual shape trace

Impedance calculation of tapered trace or multi-width trace seem use only smallest width trace (The result of tapered one and uniform smallest width was always the same)

I understand that characteristic impedance of different trace width is different. To solve this, high resistance with small footprint is added with one pad placed in place of the taper and another pad connected to GND.

Will this method give good approximation??

Impedance calculation of manual shape trace give too different result with usual connection line (same width of rectangular shape vs Cline)

Different width of manual rectangular shape trace was also tried, but calculation result was not absolutely changed.

I have no idea how the calculation input and method were used. Or program PCB SI do not support manual shape trace?

Finally, I found in help menu about RF PCB components model (Allegro RF PCB library reference >> Microstrip component) that include taper, T-junction, etc.  What program use those model for calculation? 

(My package include only Capture, PCB editor and PCB SI)

Thank you in advance for any response.

Nuttawut