I think you are a bit over-complicating trading.
Your balance currency (or cryptocurrency, in this case) is not involved in the trade until you think of withdrawing funds.
If you invest 1000 tokens on ETH/USD, any percentage needs to be calculated on that amount that you invested. If ETH/USD is up by 10%, you are earning 100 tokens.
If you are trading with a leverage of 10x, you are earning 1000 tokens. Simple as that.
Then, if you are planning to exchange MPH back to USD, you need to have a look at USD/MPH.
Consider that USD/MPH lost 10% during that period. To understand better, let's say that 1MPH was 1USD, now it is 0.90USD.
In the first case, you passed from 1000MPH with a value of $1000, to 1100MPH with a value of $990. You lost $10 if your plan is to immediately convert MPH back to USD.
In the second case, you started with 1000MPH with a value of $1000, to 2000MPH with a value of $1800. You won $800 (although your risk was much higher, because of the leverage).