I think a better method is to use the reflected group delay.
See the link about filter couplings on this page which is an article from Applied Microwaves and Wireless (May 1999)
**broken link removed**
Filters can be generally time consuming to fully calculate and optimise using full 3D solvers like CST/MWS and HFSS. You can get very close, very quickly, using circuit simulators like ADS and MWO.
Then HFSS or MWS can be used for the final small adjustments.