I have 42 year experience with a concrete based layout near Paris, France , using pre cast sidewalk borders laid on clayish soil. I had a few steps less than a 1/4" to correct over that period as roots and the clayish soil shifted a bit. As since 2019 I have moved I am currently building a new layout which is built on cinder block walls and a brick viaduct placed on real foundations about a foot and a half deep.The precast borders had the advantage that they could be leveled both longitudinally and transversly. Now my contractor has a laser level. I had to contract it out due to health reasons. I posted photos in Rolling stock > USA trains heavyweights, although they really don't belong there, apologies. I will probably move them to a new post on layout building when it is more advanced.