Self Directed 401(k) / Roth 401(k) commingled funds question

I have a Schwab self directed 401(k) account called a PCRA. It was a combination of after tax and pre tax contributions. When I buy a position, and sell it, how do I know if the PnL is post- or pre-tax? Does it do it by position or just percentage of assets contributed? So If I make $100 on a 25% pre / 75% post account, then the profit is split 25/75 or is each position somehow attached to certain contributions?