tapered stripline coupler design

I know that designing of a tapered stripline coupler can be done by designing of a stepped strip line when the number of steps goes infinity (very large).
but this is very time consuming.
how can I do this using matlab or any other simulators in link with 3D full wave solvers (HFSS or CST...)?

number of steps going to infinity sounds a lot like a linear or exponential taper. Use those equations.

yes, that's right.
but I don't know what kind of equation I should use.
I should have for example a desired S21 in the specified frequency range.
but have no idea about how to apply this parameter to the equation.