Calculating the cost of a Lot per point in a spreadbetting account

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Im writing a function to calculate my lot size based on risk and sl, pretty standard stuff so far.

Now where I am struggling is my account is a GBP spread betting account on Oanda convert everything to USD, so my question is how to I get to the price of 1 lot for my account.

So far I know Oanda convert my trades into dollars and back when placing trades and because its spread betting it is always USD but Im struggling to find how to convert to GBP for my function.

Their calculator here demonstrates

If I do the conversion manually based on the USDGBP fx ratio I get pretty close but its for a robot so should be a bit smarter than that.

Trading Calculator | Forex Profit / Loss Calculator | OANDA
Choose your primary account currency. (The tool will calculate the profit/loss in this currency.) Select the trade's currency pair from the list. Current exchange rates are shown. Choose the action (the type of trade, buy or sell). Type the number of units in the trade. Type a hypothetical closing rate for the currency pair (for example, a...
In code (MT4): Risk depends on your initial stop loss, lot size, and the value of the pair.
  1. You place the stop where it needs to be - where the reason for the trade is no longer valid.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.
  2. 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.)
  3. 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.
              MODE_TICKVALUE is not reliable on non-fx instruments with many brokers.
  4. You must normalize lots properly and check against min and max.
  5. 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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