You should choose to NOT let the machine calculate the rate and just withdraw in pesos. That way you get your bank’s exchange rate. However the fee is approx 10 USD (last I heard was 180 pesos) to use the machine so you need to factor that in. In Melaque the fees range from around 40 pesos to 200 pesos per withdrawal.

If you choose to let the atm calculate the exchange, you will lose another 10% of your money.