The number of chain lengths you use on a Reese weight-distributing hitch depends on other adjustments. Provided
the floor of the trailer was level front to rear before you hooked up, and provided the angle of the ball to the shank is correct per Reese install instructions, then you can go by the Reese rules for how many chain lengths to use.
But don't use the number of chain lengths as the final rule. You want the hitch tightened enough that the trailer floor is level front to rear when the chains are tightened. If you notice a tendency for the tow vehicle to spin the back tires instead of going on, then you have the chains too tight, so back off one chain length. If the front of the trailer is too low, you probably need to tighten up another chain length.
After you have the hitch adjusted to where you think it is right, then the next step is to weigh the rig twice on a CAT scale. Once without the spring bars tightened and once with the spring bars tightened. You also need to know the actual wet and loaded tongue weight.
The best way to get the actual tongue weight is by using a tongue weight scale, such as the Sherline Tongue Weight Scale:
Sherline Trailer Tongue Weight Scale - 2,000-lb Capacity Sherline Tools 5780
If you don't want to invest in your own tongue weight scale, then you can determine actual tongue weight by weighing the wet and loaded tow vehicle without the trailer tied on. Subtract the weight of the truck from the weight of the truck with trailer but without the spring bars tightened. That will give you within a few pounds of your actual tongue weight. (CAT scales are not accurate to closer than about 20 to 50 pounds gross weight.)
Then compute the change in tongue weight percent on the axles of the rig. A perfectly adjusted WD hitch should result in
20% to 25% of the tongue weight being distributed to the front axle of the tow vehicle,
20% to 25% of the tongue weight being distributed to the trailer axles, and
50% to 60% of the tongue weight remaining on the rear axle of the tow vehicle.
If too much weight is being removed from the rear axle, back off one chain length. If not enough weight is being removed from the rear axle, tighten up one chain length.
Then adjust the angle of the ball to the shank until both front axle and trailer axles have about the same percent of the tongue weight, while the rear axle continues to carry 50% to 60% of the total tongue weight.
Yes, it takes a lot of trips across the CAT scale to get it dialed in just right. But you'll be proud of yourself when you get it right.