Trying to get the front wheel straight and then using string is old school. The easiest way is to have the rear axle parallel to the swing arm pivot bolt. If your exhaust is not in the way, just use a measuring tape so both sides are equal.
If your exhaust or centerstand is in the way, a set of dividing calipers works well. I made a set using a straight metal pole and clamps that hold Burette tubes like in chemistry class. I put a long nail in each of the two clamps and slide them along the bar. I place one nail into the center of the swing arm bolt and the other in the center of the axle. tighten down the clamps, check the chain slack, and check the other side. When both sides have the same distance between the axle and swingarm, the wheel is perfectly straight.
This picture shows the bar, one clamp, and a long nail in the clamp with the point in the center of the swingarm bolt. The other end of the bar (not shown) has another clamp, with a long nail in the center of the axle bolt. The long nails offset the bar away from my exhausts.
