I would play around with the depth of cut depending on how much you need/wish to do. Scarifying typically cuts below the thatch layer and into the soil. Ideally you should be able to see visible grooves cut after each pass. Do 2-3 passes at varying angles. Something I always try to do is get rid of as much of the debris once you have finished the scarifying. It is really pain staking and dirty, but if you don't once you add the sand you end up with a mat layer of organic material which will stop water and nutrient flow. The best way is to run the rotary over it several times until it has sucked up all the trash.
