tipping for full-day private tour?

Six of you - I'd say 50 euros.

But you are already paying €300?

Yes, the question is how much to tip on top of the basic tour cost of 300€.

Following. We have two tours with private guides planned for May and June. One of them is just me and Stan; the other may include more people. We won't know until we get there.

If the guide is self-employed, do we still tip?

A quick google search and I'm finding recommendations for private guides for 20 Euros per person or 10-15%.

https://realgreekexperiences.com/tipping-in-greece A good rule of
thumb is to leave 10% or 15% of the price of the group tour. So, if
your tour cost 40 euro, you could leave 5 euro as a tip.

For private tours, tipping would also depend on whether there’s a
driver involved, or whether your tour has multiple stops. Again, a
10-15% of the total cost is reasonable.

If you are going on a multi-day tour, you could tip both the driver
and the guide on the last day, in the form of a lump sum.

.

http://www.whototip.net/tipping-in-greece In Greece it is standard
etiquette to tip on tours. For group tours, you can tip between 2 and
5 Euros per person, per day. For private tours, tip 20 Euros per
person, per day.

Jane, usually when someone is self-employed I figure they can set their own rates therefore no tip is needed. But clearly for the Athens guides, there is a standard rate not set by the individual guide, so I would expect to tip.

Allan, 20 euros per person would be 40% of the tour cost, or a tip of 120 euros - I think that must apply to a different situation. 10-15% sounds better to me, for a tip of 30-50 euros or 5-8 euros pp. And that aligns with Rick's guide of 10-20 euros for "a couple of hours", since this tour will be about 3 times as long. No driving is expected, and we'll plan to pay for her lunch and any coffee breaks.

I'll share this with the rest of my group. Thanks, all, for your help!

A 10-15% of the total cost is absolutely fine.