Slot-coupled microstrip antenna

i want to design a slot-coupled microstrip antenna to achieve CP operation, but if the slot is not under the center of microstrip patch ,the coupling is too small and can not excite the microstrip patch, and it is under-coupled if the size of thd slot is small, can someone tell me how to determine the length of the slot and the position of the slot? (i want to use the offset slot couple microstrip antenna rather than center slot couple microstrip antenna.)

thanks

cybcad