Cubdriver

I don't fly Arrows but the MP settings are similar in a Cutlass. You are right for noting them down so you can try adding or subtracting a little each time through. Bear in mind the gauges in these airplanes are crude and what seems right in one of them may be inches of MP off in another. Sight picture may be messing you up, and the slip skid ball may appear to be off when it really isn't due to parallax error if the unit is mounted in front of the left seat. If you are gaining height rolling into steep turns, this may be due to how the sight picture changes in the roll. The left side of the airplane will appear to descend in a left steep when it isn't, and the right side will appear to rise when it isn't. The tip about watching a chosen spot on the windscreen is ok but but you must also choose a spot that neither rises nor falls in a roll. Try putting a dot of lipstick or lip balm on the centerline of the windscreen.

Flying visual manuevers properly is a combination of proper setup followed by execution with most of your attention outside the cockpit. Use a visual reference like a mountain peak, road or ground feature rather than the DG and compass. Do not to fixate on the instruments, just use them to start the manuever, and to check progress at cardinal points. If you are really fixating on instruments try sticking a sheet of paper on them with tape a few times through, then do the maneuver without it. Commercial manuevers are designed to be challenging and are hard to do correctly even for seasoned pilots. As far as getting used to the change from left seat to right seat it takes maybe 20-30 hours to get truly used to it. You may also just be on a learning plateau which will take some additional practice to break.