I'd suggest you look into "factorial development" - essentially you time how long it takes for a particular tone to begin to emerge, and then develop the print for a total time that is 3, 4, 5 or 6 times that emergence time.
The factor you use (3, 4, 5 or 6) will depend on which paper and developer you are using, but will remain constant once you determine it.
You can arrive at the factor by working backwards - take fresh developer and prepare a print that, when developed for a time in the middle of the manufacturer's recommendations yields a good result. Then measure how long your chosen target tone takes to emerge. Divide the two times, and that will give you your factor.
Here is an APUG thread with more information: