The following are sighted parallel to the centre line so you must relate to your view angle through the spreader window. Med Inboard mark flat water. Three sets of tell tails are located evenly up the luff. As a guide medium air sheeting angle would pass through the clew and intersect the luff at half way. The car will range fore and aft off this setting from light to fresh air.
Long tabs make the sail deeper off the luff while short tabs are flatter.
The aim is to give your jibs more range in a variety of wind and water conditions. Alternatively contact Michael Coxon in Sydney, North Sails class expert on Etchells sailing.