The parametric on this amp is basically 2 parts.
The first is knobs 6 & 7. Knob 7 adjusts the frequency of the filter and 6 adjusts the gain at that selected frequency. In my opinion, the best way to adjust this is:
1) Turn the level up full or nearly full level.
2) Adjust the frequency until it is most annoying and targets the frequencies you don't want.
3) Turn the level back down until you are satisfied that the offending frequency is cut to a level you like.
The second is a simple hi Q notch filter which simply cuts a narrow band of frequencies around the selected frequency. I find that typically around 200-300 hz is where acoustic guitar likes to feed back, but each amp is different, so you can adjust it to your taste, or not use it at all. Its probable best use is going to be to cut low and/or low mid feedback, so you can engage the filter and turn the knob until feedback goes away. If you have no feedback, you don't need it. But, it can still be used to tailor the low end to your liking.