The formula is visible
Estimated decimal gigabytes = Mbps × hours × days × 0.45. There are eight bits in a byte and 3,600 seconds in an hour. The optional allowance increases that result by your chosen percentage.
Why resolution alone is not enough
Two streams with the same resolution may have different bitrates. Encoding, frame rate and the source affect the amount of data. Use a measured or supplied average bitrate when available; the default 8 Mbps is only an editable example, not a Strong 8K requirement.
What is outside this estimate
Variable bitrate, other household devices, downloads and app updates can change actual usage. The allowance is a planning input, not a measured network overhead. Check your router or internet provider's usage meter to compare an estimate with reality.