If 0% = 900 and 100% = 1800, how do you find the percentage of 1200?
Question: 1200 = ?%
ms = milliseconds
In other words, on a timeline from 0ms to 1800ms, if an object weren't to move until 900ms passed, and it would have moved exactly 0% during that time. What percent would it have moved until it stopped at 1200ms?
This formula would be used on a website :)
The percentage can be expressed as a ratio of lengths.
The longer length which represents $100$ percent is $1800 - 900 = 900$ units long, and the shorter length is $1200 - 900 = 300$ units long. Therefore, $1200$ is $\frac{300}{900} = \frac{1}{3}$ of the way to $1800$, which is around $33.3$ percent.
This is equivalent to linear interpolation as you have mentioned: the gradient of the line between the two endpoints, and the gradient of the point $(x,1200)$ and one endpoint must be equal.