I think Les McClean's book (which I think you have right?) has examples of how to calculate N+1, N-1 development etc. I believe it is roughly 20%. So, if you development time is 100 seconds, 83 seconds would be N-1. N+1 would be 120 seconds. N+2 would be 144 seconds.
But, of course, you have to test, and shorter times on the minus side don't necessarily work if the dev time is too short. 5.5 minutes N-1 would be 5 minutes (4' 58") according to that formula.
If you can use dilute developer it can be easier to get the results you want since the times are longer.
I normally expose HP5+ at 200 ISO anyway, so I would suspect that you would be OK at your normal development time as others have said.
Hope this helps,