[ohenry]

So, which coffee filter company makes color balanced filters so that you get true color balances like you would with a calibrated Expodisc (or similar device that has specifically been calibrated) ?

Your method certainly works, as does the old Pringle can lids, white card stock, and likely many other sources, and I'm not discouraging anyone from going that route. But to label Expodisc as snake oil is a bit far fetched. Professionals that demand critical color probably will like a little more assurances than "close enough".