Are we allowed to guess?
I would imagine that reference plane for the tool setting should be at the axis of rotation for the 5th axis. Now this could be difficult to find through trial and error. Would there be a machine specification for the distance from the spindle nose to the axis of rotation?
If so, then perhaps using a tool presetter would be in order, to find how far the tool tip is from the spindle nose. This would give you a series of relative offsets for all the tools. If the tooltip reaches so far as the axis of rotation, then its length offset would be zero, if shorter, its offset could be positive, if longer, its offset could be negative, or vice versa.