The euro finished the year slightly above 1.12 to the dollar, but traded below 1.09 in September 2019. So if you use the year-end figure, IRS won’t complain about collecting nearly 3% more tax. Your client might.

I use the eyeball method, for regular monthly payments. Find an annual chart and figure out where the average falls. Close enough for government work, and probably higher than what the taxpayer receives once the bank takes its cut.