Just use a tape measure. Back center to center ski, and front center to center ski. Very easy to find the center of the back of ski and the center of the front loop. Front measurement should be longer by 1/8”.
I start by sitting on the sled and looking straight ahead, and position your handle bars to what feels straight.
Get off the sled and walk to the back, stand center and then look at where your rear ski position is on the belly pan side without moving your head position. If perfect, both skis will show same amount of ski before hiding under belly pan.
I find that the straight edge from track method takes much more time, and does not yield any better results.
My bars have to be perfectly straight when riding, that is one of my pet peeves. When swapping sleds with others it really bothers me if their bars are off just a tiny bit.