I would say about 10mm. I know it doesn't sound like a lot but it all makes a difference. Also makes sure the leaf spring is well greased up. It will take a while to settle if it's new anyway.
I know it won't look as cool, but it will be more driveable and the car is a bit lower when someone sits in it anyway.
I've tried all the modes, flipped spring, lowered to the max but I'm a heavy lump so the car was often on bump stops:)
The really early 126s had lower mounting blocks fitted. If you look at the photo of them the front is much more even with the back of the car.