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."