You might check out some articles I wrote for Lapidary Journal on
etching with ferric chloride. It might be what you need, especially
the latter of the two articles, since it's concerned with adjusting
your solution to get the best results (using Baume' readings).
Low-Tech Metal Etching, June 1998; Low-Tech Metal Etching, Part 2,
July 1998; Transferring Etched Designs, March 1999. Download articles
for a small reprint fee (no, I don't get royalties from it) from