Not knowing your chassis, I would say you need a little more preload. I would try one flat at a time to see if there is any improvement. You may be hitting the rear tires alittle hard also. Your instant center may be off alittle.
As you move your actual instant center further forward or below that Theoretical "Line of Instant Center" (Neutral Line) (It is an imaginary
angled line, where you had your tape) The rear suspension hit will decrease, but the front will have a tendency to rise on it's suspension. Likewise, If you move your actual "Instant Center" back or up toward the Theoretical "Line of Instant Center", the rear suspension hit will increase.
As you move your actual or active Instant Center point forward of the Imaginary line of Instant Center (Neutral Line) you will promote more front suspension lift and less rear tire hit.
As you move the actual or active Instant Center to the rearward you will give it more tire hit and greater rear suspension reaction and less front suspension lift.
As you adjust for increased rear tire hit, you will be at the same time be adjusting from less effect of actual bar lift in the front.
When you adjust for decreased rear tire hit, you will at the same time be adjusting for more effect of actual bar lift in the front.
If you reach the Ideal, as the tires hook the car will rotate on the rear axle and will also try to lift the front, as the car is moving forward.