I asked this same question. My sister and I and 2 teenage boys went to Italy and had private guides and drivers in 3 different cities. they were all arranged from here in the US but each one worked for a different company. Since my trip was packaged as a whole inc. hotels and some admissions to museums etc I did not know the amount from which to figure a %. I gave each one 50 euro. Of course I loved every one of them but given the reaction from one or two I am sure that this was waayyy over the top. (think big 'ol bear hugs at the end of the day)The point being who knows what's right? So I think the rule of thumb is whatever your pocketbook and conscience are comfortable with is the right amount.