I tell them to punch the following into Marsha and voila, out comes the daily exchange rate.
OP/CCC/GB/100.00/US
The above can be used for conversion to from any currency. Replace GB with the origin and US with destination. 100.00 is of course the amount of the origin currency.

So far I have had success at all hotel with the above except the Renaissance Seattle. The person there just refused to take my word that the above would produce an exchange rate. To quote "how can that be? how do you know this? i do not work in accountng...." So pointed out earlier, he had an adjustment and not a credit and it took me 6 weeks to get the points adjusted.