I have some paintings to shoot coming up and will use the D70 and film. I've read the manual and it seems that setting a custom preset is done with existing light and not with strobe. I know the temps of my flashheads, umbrellas etc but the room I will shoot in has yellow walls, a warm hardwood floor and other things to throw off color. BTDT years ago and bought a color meter. So I could shoot with the current setup and do one frame with a grey card in it, then use the eyedropper in curves and hit the grey card and save the curve. Or can I set the grey balance at the time of the shoot with a preset, if I can. Can I?