IMO, most people lack flexibility to touch the ground w/ the plates when performing SLDLs, let alone off a platform. Unless you have great flexibility, the bar shouldn't be traveling much farther than the knee - maybe the shins if you have decent flexibility. You don't HAVE to go low to get the stretch in the hamstrings. Keep the back straight, knees tight. Most people, including myself, end up lowering the bar further by rounding the back or bending the knees, whether we realize it or not.
Think of it like this... if you were to lean over and attempt to touch your toes, you would probably round your back in order for your hands to actually reach your toes. This is unnecessary and dangerous. Keeping the back straight (head up) try again. You will notice you get the SAME stretch without putting your back in a dangerous position. You probably won't be able to physically touch your toes this way, but that isn't the point of the stretch. The point is to stretch the hams, and that can be done without rounding the back. Same goes for SLDLS. Don't worry about how low the bar goes... pay attention the the stretch in the hamstrings.
