Nigel's explanation is correct - it takes more time to extend the contrast from N+1 to N+2, but for initial ball park testing you can used fixed periods. You must also factor in dilute developer exhaustion for long periods of time and your agitation scheme comes into play with highlight tones. When I did my tests of Pyrocat and EMA dilution I eventually arrived at development time ranges not fixed times. N 21-23 , N+ 24-25 N+1 26-30. You can always decide to do a little expansion or a lot of expansion as you gain experience, then your decisions in the field at time of exposure become more relevant.

Things to ask yourself:
Is your subject brightness range or EV range 2 or 3 or 5 or 8 stops - what would you do differently
Where do you place your darkest tones and where do the highlights then fall - Why
Will it be necessary to expand contrast for middle tones and yet try to maintain highlights
Are you processing a single 4x5 sheet vs a Roll (8x10 equivalent, 4 4x5 sheets) and does the amount of developer in your tank need to be the same for both?
Agitation - less / more when to use which
Temperature - are 20+ minute times too long for your patience level? Maybe 72 vs 68 is better for you but what does that do to your developer (some like it hot)
What contrast do you even like in a negative - your tastes will change and it may depend on whether you wet print or scan.
Does the film you chose match your vision - would FP4+work better than Delta/Tmax edginess. 100 vs 400 speed and why?
Then you can consider the tonal separation using filters, lens flare properties due to coated not coated, fstop selection , light fall off on wider lenses, and so much more.