OK, I realize that I am in the minority here, but I have made a choice and simply stated my opinion (just like everyone else) :rolleyes:
I own and have used:
The old Kodak "computer" in their B&W dataguide.
Mr. Linden's ruler
an Ilford EM10
the enlarging attachment for a Gossen Luna pro
the algebra formula being discussed.
All of these got me close. "Close!" I still had to fine tune, and - in my experience - a test strip and/or a "work print" or two ended up being made anyway. Starting with a test strip is, for me, the quickest, cheapest and most efficient method.
Even with the EM-10 I still do a test strip...... have you priced cibachrome materials.... I would never just toss in a bigger sheet of paper and run a print like it would be a final.....