thank you very much for your kind words and for taking the time to describe the issue. I totally agree with the behaviour that you expect. At the moment, my guess is that the value you are entering in the QDoubleSpinBox gets reflected in the QSlider, which cannot represent every floating-value that you enter and then rounds to the nearest possible value, which then triggers another signal to the QDoubleSpinBox and changes its value. As for the delta increments, I honestly cannot remember what I did for those. I think the delta increment changes according to the range of the variable, the resolution of the QSlider, or maybe both (range/resolution).
At the moment, I am working on a new version of QtFuzzyLite from scratch, taking much more care on the details, and a much better design. I will address the issues you have mentioned in the new version, which I expect to release late February or early March. As soon as I have a working version, if you are interested, I could build the binaries for you so you can test it and let me know your suggestions. Otherwise, if you prefer to modify the source code of the current version, I think all you need to do is increase the range of the QSliders.
If you have more feature requests, please feel free to post them! It is a good time since I am working many hours during the week on (qt)fuzzylite.