How do I do conversions using partial BTC?

[oweaponx]

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?

I've been curious about crypto currency, and BTC. So, I'm trying to figure out how to "translate this into what I have."

To start, let’s say, to try and keep the numbers easy, that BTC starts at $7,000, where I want to buy. Later, it rises to $7,500. Still later, it drops to $7,250. I want to sell at around the $7500 mark.

Now, let’s say I’m using $20, as my start, buying $20 worth of BTC. How do I know what my equivalent will be, at $7,500. When it hits $7500, and I want to sell, how much would I be selling my initial $20 at? Now, let's say I didn't sell at $7,500, saw it dropped to $7250, and wanted to sell there? What would I be selling at, and what would my loss be, and how much would I still have made, over the initial $7,000?

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!