I fully agree with Duncan.
There are some situations however where you need the line to be nailed down in exactly the right place so you know that if one shoulder is always over it you will definitely get back safely in zero vis. In such circumstances you are better with no play in the system. My own preference (after lots of experimentation) is to carry silt screws with cable ties already attached. The silt screw is prepared by threading he cable tie's "tail" downwards through the upper end of the silt screw and out through the V-notch, such that the tail then lies parallel with the outside of the main part of the silt screw. It is then fixed in place by a couple of loops of insulation tape around the outside of the silt screw, just below the V-notch. The right amount of insulation tape is needed so that it tears easily enough to allow the cable tie to be fully drawn up, thus holding the line firmly against the top of the silt screw. Ideally the silt screw is not pressed fully into the silt so that the line is then positioned a few cm above floor level (which makes it easier to follow in zero vis and helps prevent the line from becoming covered in silt in future).
Careful positioning of the cable tie whilst preparing the silt screw (so the ratchet box is only just above the top of the silt screw) then allows one handed underwater threading of the cable tie if necessary (although two hands are needed to draw up the cable tie fully). Another advantage of this method is (if you get the positioning of the cable tie just right) the tail of the cable tie will stand up vertically after fixing. So the whole sump ends up with tails of cable ties sticking upwards - which are extremely useful if ever you let go of the line in zero vis because it helps you relocate the line again as soon as you bump into a cable tie.
The above method has the disadvantage of not being able to reposition a silt screw later on - my own approach to this is to uproot the original silt screw, leave it dangling from the line and fit a replacement silt screw in the preferred position.
The method described was developed only for extreme conditions (very low / wide winding passage with deep silt and zero vis on the return) but has been used to great effect recently in one of Peak Cavern's more challenging underwater sections. It'd be easier to show you rather than trying to describe the method; if you're ever around in the Dales or the Peak District PM me and I'll bring one for you to look at.
One other point - if you do want a loop between the line and the silt screw (i.e. not using the cable tie method) but you want the loop to be as short as possible, use shock cord. Because this is elastic it allows shorter lengths to be hero-looped around the line than if a non elastic loop were to be used. Shock cord lasts pretty well but it can always be backed up with something else if necessary. Avoid loops which are so short as to be unnecessarily fiddly if the vis disappears within seconds since fumbling may mean you don't get the silt screw on in time before blackout ensues.
Anyway, I hope these ideas help.
