James at homewaters in eugene just sent an email about a similar subject.. His tip of the month emails are very informative even though at times the subject is simple. Here is part of his email that could be helpful to you. It specifically talks about underlining a rod (i know you were asking about overlining) but the information could still be helpful to you... i hope
Cheers, Bob
Reason to undersize a line to rod ratio. Definition one has a 5 wt rod, try loading it with a 4 wt line to accomplish the following. Line weight, fly size and leader length effect presentation more than rod weight. Sometimes you need to pick up a long line and cast it back quickly (drifting in a boat) If you typically carry and cast more than 35 to 40 feet of line, you may find it easier to underline your rod. For every 10 to 15 feet added to the 30 feet in the air, you add another line weight to the load. A 5 wt line becomes the equivalent of a 6 wt when 40 feet of line is in the air, a 7 wt with 60 to 65 feet in the air. Smaller (underlining) diameter line is less air resistance but takes more line out to load the rod. Try a steeple cast in head on winds, use a high back cast and a low forward cast which has a tendency to cut through the wind and keep your loop tight. A large loop has more wind resistance.
