tIMING OF TRANSFORMED ccd MAGNITUDES

I've been producing a couple of spreadsheets to transform CCD magnitudes in B & V using the equations given by Bruce Gary. It suddenly occured to me that in getting the transformed V mag, by subtracting the transformed B - v colour index from the transformed B mag, I'm using two images taken at different (though probably only slightly different) times. So what time do I use for the transformed V mag? I can see that with a slow variable, it probably doesn't really matter but what about a fast varying nova or Delta Scuti star?

Clear (un light poluted) skies

Keith Robiinson - RKO