I'm not trying to be a smart a$$ here but there in NO such thing as the "Ideal" trolling speed. What you pull, bait or lures will dictate you speed as well as the sea conditions, down sea, down current, up wind up current. Best thing I can tell you is let you spread dictate your speed, if it looks like it your spread is pulling good then your speed is correct. Out of Wrightsville Beach, we some days have a lot of current, going into the current I may be going 4 KTs and the spread looks great, down sea, I may be doing 7 to 8 KTs for it to look right. I will say that, pulling bait look for around 6Kts for speed and go from there. Pulling lures,6 to 9 KTs or more but always let how your spread is working and looking dictate you speed. hope this helps...
in a nut shell is it! turn around and look. sometimes 7 knots is slow, sometimes its to fast, and look at the gps too. when in the gulfstream it looks like your trolling when your sitting still.