I do the same thing using a 1" diameter bar of "sidewalk chalk" instead of paint. The board thing works well but you need to ensure the tip of the nails driven through the board are exact height, (side to side) or you will induce slight errors in your measurement. Also, the tires really need to be sitting on turntables before taking your measurements. After you have scribed your lines into the chalk, or paint, the weight needs placed back onto the wheels and the wheels need turned from side to side to preload the wheel bearings and orient steering parts into their normal plane of operation. This really cannot be accomplished by simply turning the steering wheel with the tires on the floor due to tire scrub.Jack up one front wheel, rotate tire while spray painting a stripe all the way around the tire, drive a sharp pionted nail through a board, rotate the tire while holding the point of the nail against the painted section of the tire with the board on the floor for stability. Be sure the line all the way around the tire is straight, meaning hold the board steady. Do the other tire the same way. Drop both wheels to the floor.
You now have two perfectly straight lines all the way around each front tire. With the help of someone, or some good tape to hold the ruler to the tire, check the distance between the marks in front and behind each tire. I like 1/32 to 1/16 toe in at most. Done right, and if the king pins and wheel bearings are tight, you will be as close as any computer alignment machine doing this.
Keep in mind a good front end alignment starts at the rear drive axle(s). If the back doesn't track straight, you will never get the front to compensate.