What formula is VolumeForProportionalRisk using?

Good afternoon

Can you share the formula behind VolumeForProportionalRisk please?

Previously in my cBots I have used the following code...

symbol.NormalizeVolumeInUnits(riskPercentage / (Math.Abs(stopLossInPips) * symbol.PipValue / accountBalance * 100));

...But I have noticed that this isn't generally giving the same volume as VolumeForProportionalRisk.  Sometimes not even close!

For example:

riskPercentage = 1

stopLossInPips = 123.1

PipValue = , 0.00008301235

Balance = 100,000 GBP

Symbol:  NZDUSD

My code returns:  97,000 

VolumeForProportionalRisk returns 133,000

Any reason why this might be?

Thanks

Marcus