How do you know when your motor is running at the right height with a jack plate?  I have seen some boats throw a rooster tail when they are at WOT, is that for show or just the sweet spot for the motor?  I am quite a bit heavier than the person who sold me my boat and when I fished in a tourney that he was also in, he asked me why I didn't have the motor trimmed up further when I ran by him.  (It was all the way up!)  Does my extra weight warrent raising the plate some, or not.  Not something I will do by myself, I am sure the guy who fixed me up with the seller will help me out, but I just want some input from you.  

Thank you in advance!!!  

Bow weight has a tremendous affact of trim. If the guy before you had it dialed in, I doubt that changing the height is going to do anything for you. To answer your questions though:

The lower the rooster tail the better, should not be any higher than the top of motor if set up properly. The higher the rooster tail, the more force the motor the motor is applying in a downward direction and not in the forward direction.

How to adjust the jackplate. There are a few key factors you watch for. Number one is the water pressure. The water pressure should NEVER drop below the factories recommended minimum pressure when at WOT with motor trimed out or in turns. The next is to watch the tach and speedometer, if the RPM's increase and the speed doesn't, it's too high or motor is trimed up too high. Number three is pay attention to you bow lift, if you loose bow lift after adjusting, it's too high. Once you reach any of these conditions, it's recommended you drop the motor back down 1/2"

If your weight is rather significant more than the previous owner, you may have to look into a different prop that has more lift and/or more offset in the motor. Another thing you might be running into is the load balance in the boat. With just you in the boat, it may be wanting to run mainly on the starboard side and not centered on the pad. This will also kill a lot of lift.

The "sweet spot" is going to be when have good bow lift, minimum rooster tail and most likely not have the motor trimed out to the max. When right, you should be able to overtrim the motor and make it cavitate. When it does this, you can hear the motor change sounds, feel the boat/steering change and the rpms jump up a few hundred rpm but the boat speed does not increase, may actually decrease a little.

Finding the sweet spot can be time consuming and expensive.  It's done by playing with setback, props, engine height and load balance.  Once thing that helps a bunch is talking to other people with a boat and motor like yours, knows what they are talking about and have gotten theirs dialed in.