I use SLIMTs regularly for Zone System contractions. My primary films are 320Tri-X and T-Max400 in 4x5.
I find that for contractions of N-2 and greater, SLIMTs can be a great help. They can give you rather large contractions, especially in combination with reduced development, while retaining more film speed than reduced development alone.
The amount of SLIMT required is very dependent on film/developer combination and your developing techniques, so I would recommend the following to calibrate your system.
First, since I find that N-1 works well without SLIMTs, I would test for N-2, but at your normal development time. In other words, you want to find the dilution and time for the bleaching step before then processing your film "Normally."
I assume that you have some testing techniques already, but here's what I do anyway.
My first step is to expose Zone Rulers (5 stripes of one-stop increments on each sheet of film). I use two sheets to get a wide enough spread. For N-2 you will need at least a Zone 0 and Zone III exposure (I like II and IV as well) and then Zones IX. X and XI. Your goal, of course is to get the Zone X exposure to print as a Zone VIII print value while retaining enough speed to have Zone III in about the right place.
If you don't make Zone Rulers, you can use individual sheets. I would recommend shooting the first set of test negs at 2/3 more exposure than your Normal. You need to make at least three sets of these negs.
Once the negs are in hand, you need your bleach chemicals. I mix a 10% solution of potassium ferricyanide and a 3.3% solution of potassium bromide to use as stock solutions. These last forever and you don't need much. I made 1 liter stock solutions and have not used them up after 2-3 years. From these, using a small graduated syringe, I can measure out the small amounts needed to make the very dilute bleaching solutions.
I use dilutions of 0.01% to 0.04% (yes, that weak) depending on the amount of contraction. With these stock solutions you will need 1ml of each of the stock solutions per liter of working solution for the 0.01% solution, 2ml each for the 0.02 solution etc. (easy to remember this way).
As a starting point, I would use a 0.02% dilution with a time of about 3 minutes with your normal agitation, then transfer directly to your developer and develop, stop, fix and wash as you normally do. I use a pre-soak, which I recommend if you are processing multiple sheets.
After this, you can evaluate you results. There will be a marked difference, maybe to little contrast (you may have a good N-3, another good reason to include a Zone XI exposure!). Your calibration of the system will be done by changing dilution and time. I try to keep my times in the 3-4 minute range. Certainly, one minute would be too short. Use the other sets of negs to zero in on the best SLIMT time and dilution.
You will likely have to adjust your personal E.I. for each contraction as well and fine-tune the SLIMT dilutions and times accordingly.
Once you have your N-2, you can use that as an anchor to find further contractions. N-3 (and maybe N-4) should be a higher dilution with the same development time. Further contractions can be SLIMT pre-treatment with reduced development time (maybe your current N-1). I got a pretty good N-5 contraction not long ago, so this really works.
As you can see, the testing is empirical and hit or miss. I hope my recommended starting point gets you in the ball park at least. And, each film/developer combination needs calibrating.
Have fun,
Doremus Scudder
www.DoremusScudder.com
