There really isn't an acceptible amount. Variances of .1 or so are expected.
But it sounds like you aren't charging at all.

It also sounds like you have done some modifications so a generic 68 diagram might not help.

The key is voltage based on the ground.
That is to say, where you are tapping the ground for will affect the voltage you see at whatever line you are checking.
So it matters what the ground is for the item that needs to get the voltage.

So if I test the voltage reg for the charging system, I touch the sensing line(which is exposed on a factory 68 but not on the 71 and up) for the positive and the case of the voltage reg for the ground.

IF the case is not properly grounded the voltage will differ if I used the battery for ground instead.
Both of the actual postive voltage provided and the ground to the voltage reg will change what the voltage reg shows.

All of these readings should be compared to the battery directly using the battery pos and neg posts.

Also be careful on using the bolt or screw holding a wire in place instead of the actual wire as the touch point, as this too can give you a false reading depending on how good the connector is on that wire.