This needs to go in the newbie section - its been asked over and over again. The rule of thumb is 2 of droop for every 1 of compression. So if you got a 7.5 inch slip shaft, you should set it up for the running length to be with 2.5 of compression and 5 of droop.
Stock toy slip shafts work fine for rear DS, since the chunk is centered, unless you are gonna haul firewood or gravel or something that is going to super compress the suspension.