Image emergence time: the time it takes for first image appearance.
As Ansel explained it in his book, The Print, he used an area of textured grey, like grass. He was using Dektol I believe. I use a Glycin developer which is slow off the start but really picks up steam after a minute, so I use the negatives clear edge as the emergence area.
The multiple; I've read it runs 4 to 5.
Sounds reasonable as it just so happens my development factor for normal development is 4.5
The multiple applied to the emergence time equals the time
needed for complete development. Is that correct?
That's the idea.
Does the multiple vary from paper to paper or is it a safe bet that the
multiple is a constant whatever the paper?
Nothing is ever a safe bet, but it's easy to test for.
What I did was use my test negative (one that prints the easiest on grade two paper and has a full range of tones) and made the best straight print I could with fresh developer at my standard temperature and time. Then I took the emergence area's time of emergence (40 seconds) and divided that into the development time (180 seconds) which gave me a development factor of 4.5.
Now if the developer gets a little cold, the emergence time gets delayed which when multiplied by the development factor compensates for the sluggish developer. If the developer warms up, the opposite happens. You could even change the developers dilution and the new emergence time multiplied by the original development factor will give you a print dang close to, if not the same as the original.
I sometimes keep my working solution for months at a time and swear by this system for getting consistent results.
Murray