Sounds ok mate. My output does surge a bit aswell, not sure if it should but it always has. I've attached a link to a video of mine running for you to see if you like.
https://youtu.be/zfxxm4OAMTg
75% is well into tails I would think. Mine usually jumps one % telling me tails are close and because I don't go deep into tails for drinking I pretty much cut it off then. Your cuts will soon tell you where to cut it off :handgestures-thumbupleft: