Quantrocket pnl calculation

Hi
I am trying to understand how pnl is being calculated via download_pnl function in blotter in case of FX specifically cases when currency is not USD like in case of FXAUDCAD,FXAUDCHF
etc.
I looked the code blotter its making an API call to houston service but i am unable to find the code for the same.
Q. Is it possible to see houston code? If yes how?