You may be experiencing the drift that occurs in cold light heads. They can be quite unstable in their light output until the temp in the bulb is stabilized. If this is the case, I would switch the lamp on for several minutes before making any test strips determinations. If this is your problem that will go a long way to giving you more consistant light output.

In lieu of that Lee C. has worked out a closed loop system on his cold light head and it works to give him repeatable exposures. You may want to check with him on what he worked out.

I also have a Zone VI cold light stabilizer that would solve your problem too. I am not using it so I would be willing to sell it.

Good luck