Hope it ok to offer tips. For anyone who likes to to do as little post process as possible, then having accurate WB is essential. Easiest way for this is using Custom WB, as you refer to. For those that have not tried it before, I suggest assigning Custom WB to shortcut button (Print on a lot of Powershots). For a white card, personally I use a cd paper envelope I carry in camera bag. When using filters, or changing light source, all it takes is to fill LCD with view of well lit white card and a press of shortcut button for accurate White Balance on following shots. Be careful of reflections of walls, bright shirts, etc affecting the white card. Sorry if taking over your post, but IHTH, and good job on ring lights.
