A 3oz weight at 100' back will get you to 42' @ 1.5 mph, but your question said running close to the bottom so you have to remember that when turning or slowing down you will probably hit the bottom and possibly lose equipment. If you do the same thing with a jet type diver it rises if stopped and won't sink if slowed. Just something to keep in mind between two ways to get to your desired depth. I typically don't target fish that close to the bottom, and when I do it is on a bottom bouncer or a 3 way rig designed to touch bottom.