From the beginning the ProStart was designed to help sailors start better. Good starts are all about time and distance. Good distance to line (DtL) is all about good pings. Naturally, the ProStart has dedicated line end buttons on the face of the device. The right side button has a square icon and the left side button has a triangle icon. The ProStart also has a GUN button on its face for the time portion of time and distance.
Once you have set (pinged) both ends of the starting line the ProStart connects these two points in a straight line of infinite length. The ProStart uses a 25 Hz GPS (and display update rate of 4 Hz) to calculate your perpendicular distance to the infinite start line.
Whether or not you are using the bow offset, the ends of the line should be set when the bow of your boat is on the line.
When you set the ends of the line, approach the pings in the same direction you would as if you were approaching the line to start the race.
Ping the starboard or right end of the line using the square button on the right side of the ProStart.
Ping the port or left end of the line using the triangle button on the left side of the ProStart.
This technique works on upwind starts, reaching starts and downwind starts. This technique also works if the signal boat is the left end of the line and the pin is the right end of the line.
Pinging without the bow offset:
With no bow offset the DtL reading always assumes the boat is the same orientation to the line as it was when you pinged.
Without the bow offset you should ping perpendicular to the line or at the angle at which you expect to cross the line (i.e. close hauled upwind for and upwind start).
Pinging with the bow offset: 
With the bow offset the distance to line reading takes the compass heading and bow offset and calculates the distance from your bow to the line.
The ProStart calculates compass heading using a patented high-precision 3-axis geomagnetic sensor sampling at 44 Hz combined with a 100 Hz solid-state 6-axis inertial measurement unit (IMU).
Other tips and tricks:
Approach pings slowly and with a consistent course. Even with a fast GPS, like the 25 Hz module in the ProStart, we still don't recommend high speed pings.
Ping or re-ping the ends of the line for the start of each race, regardless of whether the line has moved.
Try a timed run to calculate your time and distance for your final approach.
For starts where you can approach the line perpendicularly (i.e. reaching starts), 1 knot is approximately 0.5 meters per second.
Questions? Comments? Suggestions? Send us an email, we'd love to hear from you!