Day to day fluctuations in continuous studio lighting

Hey all,

I have a studio with a couple of Westcott TD6 Spiderlites for the subject, and a bunch of light for the backdrop. I have been taking daily photos of a few subjects and I noticed that the subjects look different each day; I control all other possible light, so is it just minor fluctuations in the temperature and in the lights?

At any rate, how do I best control for this? I have a reversible white/gray disc, should I keep that in the background of all shots (because the exposure will actually change on the same day, sometimes, as well - so I don't think it would be enough to do it once each day)? I also take videos, would I leave this in the background for videos? I'd be nervous to do that, because it seems like it could be a dickens in post.

I am working with Photoshop and Lightroom, should I be working with other programs?

Thanks!