Good ideas. When I want a "professional" Looking start or finish line, I use a stencil.
Last year I was on the way to paint the start and finish for the Fairfield Half Marathon, I passed one of the numerous "raze and rebuild" construction jobs going on in the area. There was a dumpster full of scrap wood cutoffs, and I spied (and grabbed) two 8 foot long pieces of trim about 1.5 inches wide and about 1/4 inch thick.
I lay both of them down parallel, about 2 inches apart, then paint the area in between. I keep sliding the boards across the road until I've painted its width.
Not only are the lines straight, they have a very sharp edge, like the start and finish lines on a track.
Dragging the scrap wood stencils around is a bit of a pain, so I usually only do this for bigger, important races.