I have found that N+1 time is usually 25-45% longer than N time. Most films gives N+1 around +30% time.
N+2 is hard to reach with my methods. Perhaps it could be something like 150% =)
I use slosher for sheet film with D-76 1+1 at 20°C. For roll film I use normal tank and inversions.
If I would really need N+2, I would do test set with some stronger developer for N+ times.