Starting from one end of the wall, mark a cross on the ground at intervals with their centre being approximately 15mm more than the length of the sleeper.
When utilizing the fence bracket system allow an extra 20mm again to ensure the sleepers have clearance with the nuts and bolts.
For example: If you are using 1530mm sleepers the hole centres should be 1550mm apart - note, this will vary on the length of sleeper used.