Start with the amount of money
Imagine a day for which you have set yourself a loss limit of 1 000 USD. That amount was chosen for this learning example; it is not a prop firm's rule or a recommended level of risk. Closed trades have already produced a net loss of 350 USD. If the open positions hit their stops, they will lose another 250 USD, including planned costs.
If you want to keep a 100 USD reserve, that leaves 300 USD for the planned loss on a new trade. This figure depends on the state of the whole account. How attractive an individual trade looks does not change the calculation. And 300 USD is the limit under these assumptions, with no requirement to use it all.
This check covers only the daily budget. If the account also has an overall loss limit, you need to calculate the remaining room before reaching that limit as well. The new trade must fit within every applicable condition. More room in one calculation cannot compensate for exceeding another limit.
Open positions already use part of the budget
With this method, we add the result from closed trades to the full projected result of open positions at their stop prices. That is why we do not add their current floating loss to the 250 USD loss on open positions. Doing so would count the same part of the price move twice.
You can also start with current equity: the account value that already includes open positions. In that case, calculate only the additional potential drop from current prices to the stops. Both approaches can be valid, but the numbers must match the method you choose. First, write down the point from which you measure each amount.
Three trades in one account
If the full planned losses on three open trades are 120, 180, and 150 USD, a scenario in which all stops are hit produces a loss of 450 USD. Looking only at the largest trade would leave 270 USD unaccounted for.
This total says nothing about the probability of the scenario. It does not require an assumption that all instruments always move together. It answers a narrower question: what happens to the account if every stated loss condition is met? Positions in opposite directions do not, by themselves, justify subtracting risk either.
Increasing size after a loss
After a loss of 150 USD, you increase the risk on each of the next two trades to 300 USD. If they lose as well, the total loss is 750 USD. With the risk unchanged at 150 USD, those same three outcomes would add up to 450 USD.
The difference is 300 USD. The calculation does not tell you which trade will win next, and it provides no evidence that a larger position size offers an advantage. It lets you compare two possible consequences before acting. If the size change was not part of your plan, spell out the change and assess it separately.
What this calculation cannot promise
These examples assume that stops fill at exactly the planned prices and that costs are known in advance. The SEC's Investor.gov material on stock stop orders explains that the stop price is not a guaranteed execution price. You need to assess the execution conditions for your own instrument and platform separately; a 100 USD reserve cannot guarantee that every unfavorable execution outcome will be covered either.
Before your next trade, write down your personal limit, the risk you have already taken, and where costs enter the calculation. If the limit is a percentage, also state the amount on which that percentage is based. A precisely defined plan lets you see when the next trade no longer fits within it.