@Anonymiss I've always used a 600w MH's dimmed down to 400w for years. Never had any trouble.........yet.
Fair enough - I just didn't know one way or the other
@THC F13ND If you ballast allows 125w and 190w bulbs, I'd use a 150w bulb if you can get it, on the 125w setting. Maybe worth a try mate.
The label only mentions 250 Watt lamps so I'm pretty sure that's what it's designed for, and it may be that the electrical characteristics of lower-powered lamps are different and somehow not compatible with those of higher-powered lamps run below their rated output.
I'd go for a 250 Watt HPS, which I'm sure will work (and, at worst, will probably simply not strike/start at the lower settings)
The CCT will, however, probably be shifted towards the blue end of the spectrum when run at less that rated output, and slightly towards the red when over-powered.
eta: Looking at Philips brand HPS
, their 150 Watt and 250 Watt lamps seem to have the same Voltage characteristics, but only the 250 Watt versions are rated as dimmable - the 150 Watters aren't.