Why wouldn't you start by asking the question "why is the value equal to NULL?" That might help you explain why a calculation might be invalid.
If you were doing periodic updates, and the new value didn't present itself (network error for example), then NULL might mean the calculation can't be performed and the old percent value is stale. You might want to show a symbol indicating you have stale data, the update didn't occur as expected.
The same thing might be true for a missing old value, but I have a hard time understanding how an old value could be lost (unless this is the first calculation, or the battery died and the data was lost.) But you might want to show a symbol that indicates this condition as well.
In any case you don't want this to happen:
