A good starting place to determine N+1 development is to multiply your standard time by 1.4. For N+2 you can then multiply that time by 1.4 again. Differnet films and developers will of course generate different results, but this g=has worked as a very good starting place for me. usually getting me very close to my target densities. I might add that N-1 can obtained by dividing your standard N time by 1.4. This is what I learned back in the 70s while studying the Zone System with John Dowdell, coauthor of The Zone Systemizer, along with Richard Zakia.