Taking the imaginary line from 1/2 way up the luff works better with a hanked on sail. It is not so accurate with roller furling or a yankee cut where the clew may not be inches above the deck rather a metre or more.
In that case the best I can figure is that the angle of the lead should be 1/2 the angle at the clew minus the angle of the foot to the deck or the leech to the vertical which is probably the same if clew is a right angle.
More simply the line can be taken as bisecting the sail angle at the clew. The first approach will put it about 5 deg lower. Either puts you in the ballpark for then testing and adjusting the shape.
The important thing is that the lead will be rather further back than with a standard sail.
The unrolled part of the jib that will have an effect with the air flow attachment on that side. In effect if it is on the port side you will get an effect (it seems to me lol) of the attachment if any moving back lessening power. That might cause the difference.
I am not a fan of large genoas on furlers unless you sail in predominantly light winds.
There is a further factor of wind sheer differing on each tack. In the northern hemisphere it is greater on starboard tack and can require the port genoa car set further back. This was the setting initially on your boat. However wind sheer effects are greater in light winds and also close to shore. In 15-20 knots the effect would be less, and moving the car forward would put some more tension on the leech. Ocean Sail Articles: Sail Twist and Wind Shear
It is also possible that you were pointing higher on that tack because of the rolled up luff. Pinching could give you some sail shaking.