Taking a baseline that holds up later
Why the before value comes first
At the end of an initiative someone asks what it achieved. The answer needs a comparison value from before, and that only exists at the start. What is added later from memory can hardly be evidenced.
Record a benefit as a change
Record a benefit as a difference. Four hours less effort per report is a benefit, six hours per report only describes the state. The difference matters as soon as several benefits are combined. Changes can be added up and say something about the effect.
Keep the difference apart from absolute levels
For the difference you need a baseline and later measurements that refer to it. Also note the absolute level before and the target, so a reader can judge the scale. These figures are for readability and belong in no sum.
Set your share in advance
A change usually has several causes. Decide before the start which share you attribute to this initiative, and justify it in one sentence. A share set only at the end is hard to justify.
The measurement rule before the first measurement
Write down who measures, where the figure comes from and at what cadence. If the method changes, the change mostly shows the new method. A monthly cadence suits many steering measures, short enough to react and with manageable effort.
In ValueSpoc
In ValueSpoc a benefit is recorded as a change against its baseline. The level before and the target level sit beside it and enter no sum. Attribution share with its reason, measurement cadence and next measurement due are fields of the benefit, and the source is recorded with each measurement.