I'd maintain the upper curvature as you have it,but for the lower,it will depend on where you begin your diffuser.So it's angle will be 2.5-degrees,or around 4-degrees depending,and you can ignore the red line since you have so much ground clearance to begin with.
You may want to do it in 'installments',to see how you like parking 'n such.If you think you can live with more length then go for it.
