| I really don't do SLDLs that often. I do cable pullthroughs and Romanian deadlifts instead of the sldl. The SLDL should be performed with straight legs, otherwise you start running into a RDL (Romanian deadlift).
With how far you should go down, a lot will depend on flexibility, but generally, the further the better...just make sure your back is completely arched and not rounding. The bar should be kept pretty close to the body, I'd say at least within 6 inches. Keep in mind that because of mechanics and levers, each person will do them slightly different. | 
