How to avoid looping while calculating order position size in MQL5?

Question

In short, I'm writing an Expert Advisor to execute trades on the MT5 platform.

The logic is fine, but I'm struggling with the position size (lot size). Here is the idea:

1. I open a market buy order at the Current Close Price (Close[0])

2. Stop loss price = Low[1] - 5 pips (for example)

3. I want to risk only 5% of my account balance based on that stop loss

Here is my code (simplified)

int risk     = 5;

void OnTick()
  {
    if x = ...
        {

        double SL   =  Low[1] - 0.00050;   // SL price

        double Lot  =  (BALANCE * risk) / (MathAbs(close[0] - SL) * TickValue * ContractSize); 
                       // Close[0] - SL = stoploss distance


        SendOrder(ORDER_TYPE_BUY,Lot,SL,NULL,magicNumber);
        }
    }

The problem is, when I force it to open a market order, the system will require a "specific volume number", while I can only get that after specifying the open price of the buy order => order cannot be sent without a volume number. So it's basically looping with dependence error

Can anyone help me to get a predefined volume, before it is used for order execution? Much appreciated!