A silk cord is used to establish the length of the playable portion of the string.
There are several ways to establish the distance at which the string should be placed. Ideally, from a perspective of harmonic structure, the string should be placed away from the bridge exactly seven times the distance from the bridge to the point of contact with the bow hair. This will "null-out" the seventh overtone and make the open string sound as pretty as possible.
To do this, first put the bridge carefully in the exact center of the snakeskin drumhead. Then measure the distance from the bridge to the middle of where the horse hair will rub the string when you are playing normally. Multiply this distance by seven. Measure this distance from the bridge and wrap the qianjin string there.
There is another way to locate the qianjin string. This is based on making the relaxed hand automatically play in-tune melodies in the first position. If the qianjin is too low, the scale positions will be cramped. If it is too high, the scale positions will require unnatural stretching apart of the fingers.
A common way to adjust the distance is to "walk" up the stick with the fingers.
Place the thumb on the stick against the resonator box. Stretch the hand along the stick, reaching as far as comfortable with the middle finger.
Release the thumb and start to roll the index finger along the stick, staying in contact until the last knuckle is laying on the stick.
The qianjin string is installed just above where the knuckle touches the stick.
Ideally, these two ways of adjusting the string will be identical. I find, however that I have to make the string higher than the 7x rule because I have much bigger hands than the average Chinese player. A young child will have smaller than average hands, so will need a very low qianjing string.
The 7x rule is just an ideal, and the sound doesn't change too much if your are anywhere near 6x to 8x. In general, adjust the string until you can play in as relaxed a way as possible.