When to goto each difficulty

I know that normal is like 40 but Im not sure about nm. My hammerdin is at level 71 in nm atm and the exp from baal runs is getting bad, real bad. Maybe I should try for 75 then goto hell? Can somebody tell me weather I should goto hell or stay in nm for a bit longer?

For my characters...lvl 1-25 in Act 1 - 4.
From 25-50 (or even 60) in normal baal.

~50-70ish (80 if you wish) in NM Baal.

Then...Hell.

I like to do baal runs until i stop getting 3/4-1 level each run:
50~ in norm
70~ in nm

I went to hell after 74 but thx!