Frequency Planning is a highly complex subject with many interrelated variables. Getting a collection of radio based Radio Microphones, Talkback and IEM all working together in the same location without mutual interference can be a nightmare. It is far more complex than just picking different random frequencies within a band for each transmitter.
There is software available to aid frequency planning but for a job involving more than a few frequencies it is probably best to utilise the planning expertise of groups like JFMG whom you will probably have to contact anyway for licenses.
Available from the Audio Ltd website. You will need flash installed to view and navigate any of this web site. You will also need to fill in a free registration to gain access to the Downloads > Software area. Requires Win95 or NT.
Sennheiser Intermodulation Frequency Management Software. Sometimes known as SIFMPRO. This software knows all about Sennheiser equipment from the standard frequencies available to the performance of the various receivers. Windows based.
Frequency Compatibility is a simple calculator; you fill in a form with a set of frequencies and it will list those likely to have intermodulation problems. If any are found you can change a frequency and check again.