I guess I have to agree with Nancy on this. What I spend is irrelevant to what your employees should be spending. My objective is to see the country and meet the people. I tend to stay in small town and eat in local restaurants, without English menus, and with local cuisine. I spend about $27/day, lunch and dinner, drink, and tips. When I traveled on business, they put us up in 3-star (or better) hotels, because very few of us spoke the local language, and we ate, usually, at the hotel, and we spent a lot more.
You probably do business in mostly large cities and expect your employees to stay there. I noticed that Munich is almost twice as expensive for food as Garmisch-Partenkirchen, which is still a resort area and more expensive than other rural areas.
If you are only going to have one per diam amount, it should be based on the large cities, and let the employees live the life of Reilly in small towns, rather than base it on small towns and force them to dumpster dive in cities.
That said, I think the State Dept. per diam is too generous. Maybe multply your proposal by three (?).
One way to help the per diam go farther is to teach them to tip like a European. I used to round the bill up to the nearest Euro and add one Euro. When I told my hostess in Germany that, she was aghast. She told me, scoldingly, that I should only round it up to the nearest Euro. I can't bring myself to leave €13 for a €12,90 bill (10 Cent tip), so now I add 50 Cent and round up.
In order to take the exchange rate out of the equation and not penalize you employees when the dollar drops, you might consider giving the per diam in Euro, Pounds, Swiss Franc, etc. Or recalculate the per diam monthly like the State Dept does.