Although I use a colorhead for my split filter printing, the same principles apply to the use of actual contrast filters.
Originally Posted by hotami
My method is to find a basic starting time/filtration package for each brand of paper used.
So, for example, I want to find the split filter times for Bergger. I begin by dialing in full yellow on my colorhead (0 or 00 filter) and making a series of test strips (3 second increments) on an 8X10 sheet of paper at the lens aperture I want to use (in my case f/11). Then I do the same on a second sheet of paper using full magenta (#5 filter). I develop both sheets. On the low contrast sheet, I find the strip where brightest highlight shows just a hint of texture/detail. Let's say it's 6 seconds. On the high contrast sheet, I look for the darkest shadow where I want to see texture and detail and find the strip that gives me what I want--say, 8 seconds.
On my next test sheet, I expose the whole sheet at 6 seconds full yellow and 8 seconds full magenta (or corresponding filters). Then I look carefully at it to judge how close I am. Maybe it's close enough that I'll let it dry and check it afterward for drydown effect before tweaking the times. Maybe it needs another second of magenta to give it the snap I want or another second of yellow to soften it a bit, and I'll make the change immediately.
But after drying the tests, I'll be able to tell what my basic start time for that paper is. Any subsequent negatives printed on that paper at that magnification, will use that basic time (e.g., 6 seconds yellow and 8 seconds magenta) for the first work print. Usually, it will be very close on the first try.
If I want to print larger (e.g. 11X14) on the same kind of paper, I simply scale up the times accordingly keeping the ratio between yellow and magenta the same (e.g., 9 seconds Y and 12 seconds M) as a starting point.
The key is, each paper is different, so you have to test for each paper you use. But once established for each paper, your basic time/filtration will get you very close on your first try.