Estimate data savings from usage changes.
Data saving measures compound multiplicatively rather than adding up. Cutting video quality by 35 percent and then moving a quarter of what remains onto wifi does not save 60 percent — it saves about 51 percent, because the second measure only applies to traffic the first one left behind. Applying each reduction in sequence to the running remainder gives the honest total instead of an overstated sum.
Sequential compounding of savings
After each measure, remaining = remaining x (1 - saving); saved = original - final remaining
Almost always dropping streaming video from 1080p to 480p, which cuts that traffic by roughly 70 percent. Since video is usually 60-80 percent of a mobile plan's total, it dominates every other measure.
Because they apply in sequence to a shrinking base. Two 50 percent savings leave 25 percent of the original, not zero. Adding them would predict eliminating all usage, which is clearly wrong.
For a metered mobile plan, yes — the traffic still happens but no longer counts. On a capped home connection it is not a saving at all, since the wifi traffic goes over the same metered line.