1)
DI Angle φ, Inital Knockback Angle ψ, De-facto knockback angle α; counterclockwise, one rotation^=2*π
α=ψ+(sin(φ-ψ)*(π/10))
Means that you will get a π/10 (=18°) angle change with perpendicular DI. The nearer you DI towards your initial knockback direction or its opposite the less influence it has.
2)
Brawl SDI counts diagonal input as sum of vertical and horizontal input (so it gains 2^(1/2)~1.414 times as much distance). In Melee/PM diagonal inputs count as sum of the vertical and horizontal input * (2^(1/2)*(1/2))~0.707 so the distance is the same.
3)
Try to get perpendicular DIs down and know when do you want what (sometimes the maximum vertical or horizontal distance etc...)
Hope that helps despite it contains math