I, me personally have found with a Running Lead fish lines as slack as possible.
If you are fishing over silt try to find out what weight lead just sinks into it, you can then get away with a lighter lead than normal.
With Semi Fixed Rigs you fish a tight line to increase the Bolt effect, but if a fish just mouthes the bait and gets away with it, you may get no indication whatsoever, yet a Running Lead will give a bleep or other indication.
As for Rigs also have a look at Complicated Rigs, we had many thoughts on Leads, Rigs, and hooking arrangements.