I added a second site which then made the Data Decode Threshold available to modify. I just left the Freqs and TGs empty.
If you had said you added an additional site in the programming and assigned a different site to that duplicate, I'd say what you did makes sense.
However, if I understand what you are saying, if you only have a signal site for a given system, it really means nothing to modify the settings - these are intended to tell the radio to look for a better site(frequency) if the decode doesn't meet the settings. In your case, it sounds like there isn't going to be a better site to look for.... This is why it isn't an option to make changes for a single site system.