Ive commented a few times in threads about offsets.
Usually along the lines of:
"there are a lot of variables like;
what rim diameter and width will you be using?
what model Datsun?
sedan or wagon?
what struts and hubs are you using?
do you want to lip the guards or leave std?
Do you have camber plates in the front or camber correction in the rear?
What ride height and will you be using and are you using extended lower control arms etc."
These all affect what rim offset you go for as some might tuck and be fine where as other might not and either hit things or hang out past the guards for an instant defect when a cop sees you out and about on the street.
On a standard 1600 I liked the factory 15x6" R31/sigma turbo rims. They are all between +25 and +30mm.
Recently on my wagon ive fitted 15x7's with +25 at the front and +18 in the rear with only mild lipping of the front guards using lowered 240K struts and no camber plates or ext control arms.
This is on what I call std offset datsun struts i.e. 1600/180B/200B/240K etc.
R31 struts can have slightly more +'ve offset rims.
280zx struts can have slightly more -'ve offset rims.
Have you seen this thread?:
http://ozdat.com/forum/viewtopic.php?f=51&t=16561