Hi,
Do you know what kind of 250W MH are you using? 10K? 14K? How old are they? SPS colouring is also dependent on the temperature of your lamps.
IMHO, the Alk test from Hagen is alright. Its not wonky like their calcium and nitrate (according to Hagen...I have 0 nitrates....but I also have a calcium level of 700ppm
) As long as you are not demanding the absolute accuracy and you just want to know if you are in the right ball park, its okay. Its probably one of the few test kits that kinda work properly from Hagen.
Good luck.
- Victor.