Here's some often overlooked and misunderstood value stream mapping math.
The lead time ladder has two levels or “rungs.” The bottom rung is the process or processing rung on which the relevant process time is dropped down. This is usually pretty straightforward...unless there is a split or branch in the material, service or knowledge flow within the value stream.
For example, after a fabrication process, the material may flow 35% of the time to a coating process and then to an assembly process, while 65% of the time the coating process is skipped altogether. The value stream map essentially reflects a process time for a hybrid (or weighted average) product or service, in such a situation. Accordingly, the process times for these branches must be apportioned by the split %, as illustrated in the figure below.