Distance should be as close as possible for maximum efficiency.
All the friction is inside the support boxes. The support bars are held in place by the springs and bearings inside the support boxes, under pressure, resulting in friction between the support bar and sliding surface. It takes some force to overcome this friction force and open or close the joint. A D160 only has one set of springs and bearings inside the support box, so very little friction to overcome. A D400, for example, has four sets of springs/bearings at each box location, and as a result has much more friction to overcome.
In theory, you should never need more than one tool.
You should place the tool as close to a support box as possible, because this is where the friction is, and the closer to the box you get the more efficient it is.
By slowly moving up and down the joint, you adjust the joint width to the desired opening at each box location. Obviously if you have more than one tool, you can speed up the process by moving the joint at more than one spot simultaneously. For bigger joints, a temperature adjustment device can be very beneficial, and worthwhile.
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