Each weapon will have a different scan area usually. As of right now, I have no way of determining where the crosshair is at any given point of time. To find the proper value, do the following
Go in game, pull out the weapon you want, and go into marksmanship mode.
Take a screenshot
Open a program that lets you measure a rectangle in pixels (GIMP, Photoshop, etc.)
Measure the distance from the center of the vertical crosshair to the left edge of the white in the right half of the horizontal cross hair (add 1 or 2 pixels) Very confusing i know... lol
This will be the start value.
Repeat the above but while moving, and this will be your stop value. However, you might want to reduce this value to something more reasonable. Anything over 20-25 and you'll be spraying and not hitting anything at all.
Personally, a DMR for me on a 1920x1080 screen is 5 start/5 stop
You want to keep the start and stops the same on a sniper rifle.