A withdrawal on a VISA card denominated in USD will be converted from CLP (during the entire today’s banking day) using yesterday’s closing rate, and this is the currently showing rate by the official visa calculator (on the US visa website) for today.
I have verified this several times, and it works exactly, to the best of my knowledge, so today’s daily fluctuations don’t matter, and the rate is fixed. Today’s withdrawal rate will be about 872.5 (yesterday’s closing rate). Tomorrow’s rate will be today’s closing rate.
This is just my personal experience, no guarantee.