There's no need to work out the TRX-to-Stars conversion by hand — checkout does that automatically, using the live rate the moment the order is placed.
A card payment shows one fiat total with nothing to convert; paying with TRX means reading a converted amount, though checkout works it out automatically either way.
The TRX amount shown for a Stars order is calculated the moment the order is placed, using the live rate on TRON.
No — checkout calculates the TRX-to-Stars conversion live, using the current rate at the moment of ordering.
The formula is the same, but the rate updates continuously, so the exact TRX amount can differ order to order.