Not an expert but an amateur purchased to install a 3 m rail. The distance is 45 cm. Fixing is 4.2 mm hole, one dish screw one point. Since it is a structure that mounts the rail just above the fixed screw part (lower?), Positioning can be done while temporarily assembling the metal fitting and the rail, and it can not be done as it is fixed as it is (from the manufacturer it is new with the W stay There seems to be a ceiling metal fitting that has never been seen). It is very difficult to scribe and measure the dimensions and dimensions as much as possible, but it is very difficult to scribe a length of 3 m straight on the ceiling, even if the brackets are fixed, the rails will not allow the rails to do so (that is the strength · There will be precision) Fixed several times. I struggled while installing it admiring that professional skills and experience are incredible. After completion, the metal fittings and rails are strong and sturdy enough to think that you can do suspension. If the screw of the mounting screw and the taper condition do not make it perfectly flush with the chitin, the rail can not be mounted tightly with a few millimeters of protruding commutator, why also here? I suffer for a while on a stepladder. The course thread jumped out a bit and it was useless, the traditional dish and wood screw was perfectly flat and perfect. I think that the layman does not do this installation many times, but for those who install this rail from now on, the S bracket provisionally assembled the rail if it could be installed from the wall It is recommended as it can be aligned → installed.