Actually, though Farmer's Reducer as packaged by Kodak isn't intended for proportional use, if you don't mix the solutions of the two packets it can be used that way -- as can homemade Farmer's Reducer. The difference seems to be in whether fixing of the silver converted by the ferricyanide takes place as it converts (cutting) or afterward (proportional).
Given the prices of ferricyanide and hypo crystals, I'm not sure why anyone would buy Farmer's Reducer (especially since the components are so useful separately), but if Kodak doesn't still sell it, you can certainly get it from Photographer's Formulary.
Like Donald said, it is much better to buy the potassium ferricyanide in bulk. You then have the freedom to mix various dillutions (such as one for detail work and one for print imersion) and only mix the quantity needed at the time.
I did a workshop many years ago with George Krause, the master of chemically torturing both negatives and prints.
George's approach bleaching prints was to mix a small batch of bleach when and as needed from potassicum ferricyanide crystals. With the print still in the fixer, he would take a small amount of fixer directly from the fixer tray in a small container (a shot glass, a one-ounce "graduate", or one of those disposable plastic condiment cups from the fast-food place), add a few crystals of pot ferri until the color was right (about the color of weak tea, or as someone said here, healthy unine). Then he would take the print out of the fixer, drain it and blow away the surface moisture, and paint the bleach onto the print using either a small brush or a cotton swab. Then he would drop the print back into the fixer to stop the bleaching action. He would repeat this process several times until he achieved the reduction that he was looking for.
George would also bleach or intensify negatives as he felt they required that kind of torture.
It's really interesting to watch a master printer work!
This is how I use Farmers Reducer. I buy the Kodak product and mix it in two small bottles until I need it.
When I get a print that's too dark I slip the whole print into a tray of Reducer and lighten it up a bit. Now sometimes I get a print that the white is just too dark... like a cala lily on a black background. So what I do is, I take some cotton balls and apply the Reducer to the white area to lighten it up.
Of course this happens too slowly for me so I get frustrated and put the whole print in the tray. Then I leave it in the tray too long and totally bleach out the white areas, ruining the print.
Then I find the negative, reprint the photo lighter, and throw away the print that I over lightened.
I must enjoy using it this way since I've repeated this a number of times and have become very skilled at it.
Ansel Adams gives formulas for Kodak R-4a (Farmers Reducer) and for Kodak R-4b (Farmers Reducer, two-bath version) on pages 257 and 258 of "The Negative." R-4a is a "cutting" reducer, meaning it "affects the low values first" (p. 237.) R-4b is a proportional reducer and is thus recommended for reducing over-developed negatives.
Either can be used on prints. If you are selectively bleaching small areas, R-4a is probably the way to go. The issue with prints, however, is how to control the bleaching to prevent over bleaching. The easy way to do this is to add more part b than part a, up to 4 times the amount. It slows down the bleaching effect. You want to mix uup very small amounts of the reducer, as it only lasts about 10 minutes.
Lambrecht and Woodhouse discuss their method of selectively bleaching prints, and give a formula for "ferry" on p.218-9 of "Way Beyond Monochrome."