I'm guessing you have a 69 bug?
Theres no direct guide line, but the best thing would be to narrow the max 3", because you almost always need at least 2" narrow, that way you can space the rim further out with spacers. Spacers obiviously are easier to replace than having to narrow your beam more.
Do you have dropped spindles or a disc brake kit? As these things will space you wheel out further and you get their benefits. I'm getting a drop and disc kit and I'm hoping not to have to use spacers.
For more info, try to search here on the forum to see what others have done. And definately check here:
http://www.vdubcustoms.com/Tech/tech_wheel.htm
Really good info on how to approarch it, plus he's got great brake kits if you havn't started yet.
Rip