Calculating lot size as a function of account balance

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Before I start, I know this isn't neccesarily the way people usually do things. But I am trying to write a small piece of code that will calculate a lot size that will enter a trade using a given percentage of the available balance (including leverage) in my account. I can obvously do this during manual trading because my broker tells me how many units are available and I can just work it out with a calculator. But I want to automate things, and I don't want to just use a set volume, because as the account grows (optomistic, I know) I will be able to make bigger trades, and so I will have to manually change the lot size. Trouble is I'm getting a bit confused with the AccountEquity and AccountBalance and AccountCredit and such. Can anyone offer any assistance?
 
Risk depends on your initial stop loss, lot size, and the value of the pair.
  • You place the stop where it needs to be - where the reason for the trade is no longer valid. E.g. trading a support bounce the stop goes below the support.
  • Account Balance * percent/100 = RISK = OrderLots * (|OrderOpenPrice - OrderStopLoss| * DeltaPerLot + CommissionPerLot) (Note OOP-OSL includes the SPREAD, and DeltaPerLot is usually around $10/pip but it takes account of the exchange rates of the pair vs. your account currency.)
  • Do NOT use TickValue by itself - DeltaPerLot and verify that MODE_TICKVALUE is returning a value in your deposit currency, as promised by the documentation, or whether it is returning a value in the instrument's base currency (EUR, in this case).
              MODE_TICKVALUE is not reliable on non-fx instruments with many brokers.
  • You must normalize lots properly and check against min and max.
  • You must also check FreeMargin to avoid stop out
Most pairs are worth about $10 per PIP. A $5 risk with a (very small) 5 PIP SL is $5/$10/5=0.1 Lots maximum.
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