The measurements of the downhaul deeply affect the tension and the overall luff curve of the sail, so it is very important to adjust the optimal settings for your sailing session.
When designing sails, manufacturers measure the downhaul from the bottom of the mast to the perpendicular projection of the bottom corner of the sail pulley. That information is then printed in the sail.
This means that each sail and windsurf brand has its own measures. Although the differences might not be too big from brand A to brand B, you should confirm your downhaul specs on the sail and adjust hit properly, before hitting the water.
In fact, half a centimeter makes a difference of 20-to-30 kilograms in the sail. At intermediate and professional levels, it is basically critical to get a fine-tuned sail.
The downhaul is one of the most important steps to rigging. It will control the shape of your windsurfing sail and can't be changed while sailing. Finally, confirm that your tack pulley is at the bottom of the base pulley.