Grass level with border

Hello All,

First time poster so please go easy, .

I am just in the process of re-turfing and re-flagging our back garden. We are going create a border using railway sleepers around the perimeter of the turfed area, due the garden being uneven.

What i am struggling with is trying to find out what distance to leave between the height of my soil and the height of my sleepers so that when i lay my turf it is perfectly level with the sleepers?

Do i just leave it the thickness of the turf lower? Or do i allow less to allow for compensation for settling?

Any advice would be greatly appreciated.