How much should I tip a private tour guide?

That would depend on whether the private guide is an employee of a tour company.. or a independent tour guide.

I go anywhere from 10 to 25% depending on how engaging the guide was.

It depends on how much you liked the tour. Is it a walking tour, a bus tour, or a mini-group tour in a van. Has the guide had to do any special research just for your tour to cover a specific theme?
Lots of different variables to consider. Perhaps give us more details about your tour? How long, how many people, theme, and cost.

you don't need to tip at all,if it is a private your you have agreed a fee for the tour and as far as I am concerned that should be it..

I'm with Pat and Uncle Gus here.
If you have booked thru a company then 10% + depending on how 'good' the guide is.
If you have booked him independently and directly, then I really think a service premium would be included in their quote, and would be surprised if such guides expected anything.

Yet, when a bartender spends 10 seconds pouring you a beer which also has a fee/price, you don't blink an eye about tipping them. What about a taxi driver who has an agreed fee, or a hair dresser, or a server? They all have agreed fees, yet everyone tips them.
A guide who has spent considerable time and effort researching and learning their subject also appreciates a tip. Of course you don't have to tip, but it certainly is a nice gesture.

it is Europe not the US and we don't tip bartenders , taxis are usually rounded up to the nearest £ or Euro and I don't tip hairdresser, well I don't have much hair anyway.
Tipping culture in the US is very different from Europe ,nobody gets upset if you don't tip them and in the service industries folk get paid a salary and don't rely on tips to get their pay.

Perhaps an independent guide would quote lower for a New Yorker, thinking they may tip, and higher for a Scotsman, knowing they may not. (mocking my own Scottish heritage here).

good suggestionDavid. BTW we could be related I have an Uncle living in Redcliff Brisbane.

Well, I live in Europe too and I do tip my hairdresser, the housekeeper, the bartender, the server, and the taxi driver. It isn't a lot by US standards, but I do tip. I go on tours all the time and always tip the guide if they have done a good job.

yes but you are an American living in Europe still sticking to the way you do things in the US.

I've only ever had a spectacular tour guide when in Tallinn--I used EstAdventures for 2 small group day trips outside of Tallinn, it was late in the season and the first trip was just me, another couple and the owner himself and we had a fabulous time. I tipped 20 euro. The next trip it was just me and him and he went out of his way to take me to a wonderful knitting cooperative in Haapsalu and it was like spending the day with an old, comfy friend, so I bought his lunch and again tipped 20 euro. The least I've ever given a guide is 5 euro.