Size isn't everything

The handles on the front of ARX amplifiers protrude forward from the front panel a distance of 28mm. However, the amplifier front panel is 4mm thick in the handle/rack mounting area (3mm steel panel plus 1.2mm reinforcing bracket behind it).

Thus the distance forward of the rack mount stripping needs to be a minimum of 32mm if the handles are not to stick out past the front of the rack.
Of course, it is possible to remove the handles, but this leaves the level control knobs protruding 25mm by themselves with no protection - potential trouble for sure.

ZA series multi-channel amplifiers and Cinema Pro series amplifiers are supplied as standard with rear mounted level controls; for these models, removing the handles may be an option if the available space is limited. If you are doing so with these models, allow 12mm from the front of the rack stripping (7mm for the power switch and 4.2 for the front panel).

More space out the back

On the rear of the amplifiers, two extra rack mount ears enabling four-point-racking protrude 25mm from the rear panel. These are spot welded to the chassis for maximum strength and can not be removed. We recommend that all models of ARX amplifiers be racked up using both front and rear mounting points.

In keeping with current world safety regulations and standards, all ARX amplifiers have 4 pole Neutrik Speakon connectors for the speaker outputs.
Large, rugged and heavy duty, these connectors have further specific space requirements.

Once plugged in, the actual body of the line connector protrudes 50mm.
If the Speakon chassis connector on the particular amplifier model is screwed to the amplifier chassis from the outside, then 7.5mm must be added to the 50mm.
If the chassis connector is mounted from the inside, then you only need to allow 2mm.

But wait, there's more!

Space must also be allowed for the actual speaker cable coming out of the end of the Speakon line connector.
Depending on the thickness of the cable, allow a bend radius of, say, 10mm for thinner cable, and 20mm for heavy duty rubber sheathed cable.

So, as a general rule of thumb, a minimum of 80mm should be allowed for from the rear panel of the amplifier to the rear edge of the rack.

Adding up all these figures, the amp rack should have a minimum front to back depth of 32mm + 405mm + 80mm = approximately 520mm.