Save Effect Preset in Final Cut Pro
Although you cannot change the default parameters when applying templates in Final Cut Pro; custom effects presets can be saved as a way to quickly reuse preferred parameters.
Apply the desired effect to a clip
After customizing the effect parameters in the Video Inspector, in the lower right click "Save Effects Preset"
Title your preset and select which attributes you want to save to the preset
If you regularly use this effect with these parameters, consider setting it as your default effect