I have Tein SS coils.
I can tell you that the washer goes under. I made that mistake when I first put mine in, luckily it didn't go through. After re-assembling them quite a few times trying to go lower, I thought about making some sort of little cover that goes over the back strut tower that way you could just take it off and unbolt the coilover or adjust the dampening if need be. Something I wanted to do, but never got around to. Might be an option for you though