What's the best method to chalk a line parallel to a wall?
"Parallel to a wall" is a subjective thing. Parallel to what? Parallel to a line crossing the corner points? Parallel to an average wall line? 

Generally, a carpenter takes one of two approaches: Either measure out from each corner, taking into consideration anomalies such as rounded plaster or drywall, and use that. Or, if there's a nearby perpendicular wall that's considered more trustworthy, use the 3-4-5 technique to strike a right angle from that, or from a line snapped parallel to it. 

The key to any such process is getting as wide a sample as possible. Minimize extrapolation (extending your chalk line beyond measured or calculated points) and use as much length as you have available to make your line marks. 

In the case of your flooring, it may be more important from an aesthetic perspective to achieve the illusion of parallelism, and not true parallelism. Consider your entire scenario and find the best compromise. Chances are that you'll have to rip the edge plank to fit a wavy or crooked wall regardless, depending on the thickness of your base trim, etc. 

Be sure to measure across the room and determine how parallel you'll be with the opposite wall. If your flooring will extend into other rooms, examine those as well.