Tipping - what's appropriate for a private tour?

How much are you paying this guide for the tour? We did a fantastic 4 hour walking tour in Berlin recently with a University student. I think from memory it cost us about 25 euros for the two of us. In that instance we felt a tip was a fair thing. On the other hand, one of our day trips cost us $280 with a professional company guide. No tipping on that occasion!!
Tipping is not part of the culture in Australia. Isn't the rule of thumb 10%???

we are with our guide for about 8 hours and going to a museum in Naples and to two sites. fee 400 euros; we will host our guide for a nice lunch between sites. just wasn't sure what tip was appropriate under those circumstances. 10% is what I'd thought, assuming a tip is given under a package deal such as this...

If I was paying a guide this sort of money and buying him lunch into the bargain, I wouldn't be giving him a tip.

I tend to agree with Judy - lunch is the tip in this case. We gave our private tour guide a 5 euro tip on a 100 euro payment for 2 hour tour with pick-up/drop-off from the train station. He seemed perfectly happy.

that's what my husband says too...lunch is the tip. He was reading Rick's comments and that was his take--the guide is working for himself--there's no middleman.

Assuming you are paying him directly and a cut is not going to an agency.... Then if you don't tip him he will be making $72 per hour. How much more than $72 per hour do you think his time is worth? If you think $72 per hour is too low give more. If you think $72 per hour is fair or better than a tip is not needed. Keep in mind that is about one and half times what a medical doctor makes in Italy.