How does this differ from my original answer of D=0.7(posted 8725 days ago)If you read Adams' "The Negative" you'll get most of the info you need, but basically a negative exposed as a midtone (zone V if you like, 18% grey if that's what you think a midtone should be) should have a density around 0.7. The exact density depends on whether you're using a diffuser or condenser enlarger, since it's zones in the final print that matter, but if you don't end up with D between 0.65 and 0.75 then it's likely you're doing something wrong (or at least something odd).
Manufacturer's suggestions for "normal" development times are sometimes so far off that it makes you wonder just how they DO decide on them! Technically I assume they develop to give the correct gamma and toe point for the ISO specification rather than aiming for amy particular "midtone" density.