I apologize up front for the horrible title, I do not have the mathematics vocabulary to eloquently summarize this in a title.
This first picture and question is a lead-up to the actual question. In an attempt to explain it.
You have an object of a known size, you put a mark on it at a certain percentage of it's size. You now increase the size of that object in one specific direction, you know by what percentage that object changed size, at what percentage is that mark currently located?
That's a fairly simple answer. Now knowing the previous question, consider the following:
You have an object of an unknown height. You have a mark that has a known height as a percentage of the height of the object it takes up and a known location on that object as a percentage of the height of that object. You increase the height of the object by an unknown amount. The only thing you now know is the height of the mark as a percentage of the height of the object. How do you find it's location as a percentage of the height of the object?
Edit: The green object ONLY expands down, and the red mark stays in the exact same position relative to the top o the object. The red mark also maintains the exact same size in units, even though that size is unknown.
I hope that made sense. If that is unsolvable, what other variable do I need? (Additionally, what are appropriate tags for this question?)
Edit: Oddly enough you can intuitively solve this when the numbers are simple. If the red mark is at 50% of the height and the green box increases in size downwards by 100% of it's current size. Then the red mark will be at 75% of it;s height. I'm not sure if this is an intuitive guess that just ends up being correct, or if there is some sort of mathematical backing.