The angle of coverage provided by a zoom flash is conventionally referred to in 35mm full-frame terms. That means that if a flash is set to the "50mm" position, the zoom head projects a beam wide enough to cover the field of view of a 50mm lens on full-frame. However, the crop-factor must be applied in reverse here — if you use a 50mm lens on a Pentax APS-C camera, you're losing power if you set the beam to be that wide, and the 35mm flash setting is actually appropriate.
The Pentax and higher-end Metz zoom flashes can instead display numbers that match real APS-C focal lengths, so you don't have to think about this.
Of course, the other automatic-zoom flashes, like those from Sigma, actually zoom to the correct angle of view for APS-C cameras, but their displays (electronic or mechanical) require mental conversion. (Or, some models don't even have displays, making that moot.)
