This is the answer I got on a different forum:
That’s an interesting question. To answer it, it’s crucial to understand what stop-out level is and how the formula for checking whether it was hit works. For the sake of simplicity let’s consider that your account is in USD, not in EUR.
Stop-out level is the threshold value for the account’s margin level when all trades will be closed. Margin level is calculated as Equity / Used Margin.
For example, if your account’s stop-out level (set by the broker) is 30%, then your trades will get automatically closed when Equity / Used Margin = 0.3.
And what is Equity for the purpose of your task? It is the equity that will be in your account when your trade reaches the stop-loss, so it’s Account Balance - Position Size * Stop-Loss * 10. Or for your example, Equity = 300 - PS * 50 * 10, where PS is the position size, which we are trying to calculate.
Used Margin is (PS * Contract Value) / Leverage. For a EUR/USD trade and USD account, the Contract Value depends on the current EUR/USD price, let’s use 1.09. Then For your example, UM = (PS * 109,000) / 500.
Now, if we combine everything with the margin level formula, we get:
0.3 = ((300 - PS * 50 * 10) * 500) / (PS * 109,000)
0.3 * 109,000 * PS = (300 - PS * 50 * 10) * 500
32,700 * PS = 150,000 - 250,000 * PS
32,700 * PS + 250,000 PS = 150,000
282,700 * PS = 150,000
PS = 0,53 - this is in standard lots.
In general, the formula for quick calculation (which you can create in Excel for example) is the following:
PS = (Balance * Leverage) / (Contract Value * Stop-out Level + SL * Leverage * 10)
Important note #1: “10” is the value of 1 pip for 1 standard lot.
Important note #2: If your account is not in USD, two adjustments have to be done. First, the contract value becomes 100,000 (for EUR/USD). Second, that “10” becomes “10 divided by EUR/USD Bid rate”.
Important note #3: If you are trading some other currency pair, all the necessary conversions have to be done accordingly. For example, if you are trading AUD/NZD, the contract value has to be adjusted (because it’s 100,000 AUD) and the pip value should be adjusted (because it is 10 NZD).
I hope this makes sense.
EDIT: Important note #4: If you have other trades open in your account, this becomes quite complicated and somewhat unsolvable.
So I got no idea what they meant here. How is one supposed to calculated in the Position size without knowing what the position size is beforehand… i was never good at maths