Personally, after knowing that both fluorescent and HID lamps need ballasts to operate at certain currents, I generally treat HID lamps the same as fluorescent lamps. Whenever I find appropriate ballasts for them, I make sure they start reliably, run within 10% of their rated current, and run them on ballasts that can tolerate their arc voltage drops appropriately. Does anyone else treat fluorescent lamps and HID lamps in the same manner too?

Well fluorescents are way more tolerant towards the current variation, because as cold running (not more than 100degC above ambient), the temperature is less sensitive on the real power dissipated. So may tolerate way lower input power before losing performance coefficients (efficacy, color properties,...).

I just run lamps on their correct ballasts. If I don't have a ballast for a lamp, then I get one.The only exception at the moment being the ballast for a 200W SLI/H lamp, which seems to be unobtainable now 

A 150W HPS ballast seems to be the closest match, but I never run it for long, two or three hours or so at the most 

Hello,
If I have a Lamp i do not have the proper ballast for, I calculate the ballast impeadence needed. When I have that, i find an good ballast combination for that impeadence.  ON HID lamps I would never tolerate mor the 3% difference to the noted nominal data for new lamps voltage and current, I use therfore my PowerLogic PM5100 monitor, as it was way cheaper then a proper power analyzer, however it is a nice bit of kit. For used lamps, that might be adapted according to their state, and how specs drift during a lifespan.

For Fluorescent I do it similar, however im a bit open to tolerance, however more then 5% difference to nominal, I do also not like to use.


Best Reguards

Alex

Ah well, I wouldn't have a clue how to go about calculating the impedance required, that's a bit too technical for me. But whatever works for you, if you can do it, go for it 

I wonder if there is a way to calculate the impedance of an autotransformer ballast.