My rig length does vary on the baiting situation, i.e if i am fishing pva bag's or over a large bed of particles i will use a short hooklenght 3-6" long,because I have created a situation where the carp is more likely to move a short distance to the next mouthfull.But if I am baiting over a larger area with a scattering of bait's I will use a longer hooklength, i.e more of a confidence rig, as in these situation's baits can be more individually checked out, and the fish are more likely to pick up the bait and move off a distance in search of its next meal.
The lake bed also has an influence on your hooklength, I.E very soft silt "may" require a longer hooklength so that your hookbait is not dragged into it too far.
But try too make sure that the situation you are fishing in, or have created, is the parameters for your decision making
This may be use to someone cheers