For currency conversions, the IRS allows you to use any reasonable method. If you know the average exchange rate on the day of the deposit, that's a good one to use.
It is also acceptable to use the IRS annual average rate, found HERE. Since this is an information report and no tax is actually due, it's not as important to find the "best" rate for your situation, so I would just go with the rate that is easiest to obtain.