Would I use rations, or something else to solve this?

Oct 2019
Greetings, all. I’m not exactly sure how to word this, so hopefully this makes sense. I’m trying to figure out how to set this up, maybe a ration? P&L?

Let’s say I’m looking at a REIT, or maybe a fund. However, instead of buying the full amount of the item, I’m looking at a part.

To start, let’s say, to try and keep the numbers easy, that the item starts at $7,000. Later, it rises to $7,500. Still later, it drops to $7,250.

Now, let’s say I’m using $20, as my start. How do I know what my equivalent will be, at $7,500. Then, as it drops, what will my equivalent be, when I want to sell. I’m also looking to do similar, with a start of $50, and $100, as examples.

I hope this has made sense, and I appreciate help!