Keep in mind that the Mystere is infamous for having a bad rudder system. My son sailed one in the Worrell 1000 and renamed the boat "Mis-Steer."
I had one years ago and the rudders were all crooked after they came out of the mold. This makes for lee helm on one tack and weather helm on the other.
Just take a good luck and eyeball the actual rudder blade. You might just find the rudder blade is not true.
The other major problem I had with the rudder system is it kept kicking up on deep water for no reason.., and then when I came to the beach they would stay locked down.
Another Worrell 1000 war story is the Mystere that hit the beach for the finish and instead of the rudders kicking up, the stern tore off the boat.

As for how far under the boat your rudders are, you need use a straight edge on the stern of the boat and draw a line on your rudders -- both rudders.
The percentage of the rudder that is forward of that should be the same on both rudders. If not, that is one of your problems.
I can't recall what the best percentage should be, but around 15-20% or so of the leading edge of the rudder should be ahead of that line.
If it is too much, you will get some pretty radical rudder reactions -- lee helm, then suddenly weather helm.., than back to lee helm, etc.
If it is too little, you will have lots of weather helm.
Good luck,
Rick